How to Calculate Your Koi Pond's True Water Volume

The Short Answer

For an existing koi pond, the most reliable practical method is usually to measure the water added during a controlled fill using a calibrated or trustworthy water meter or totalizing flow meter. Record the amount required to bring the complete circulating system to its normal operating level. That figure becomes the best estimate of total system water volume.

For a pond that cannot be emptied, calculate the geometry carefully, include the water held in filter chambers and pipework, then cross-check the result with another method where practical. A controlled salt-concentration change can be useful as a verification tool if salt is appropriate for that system and the salinity instrument is accurate.

Keep at least three numbers in your pond records: the pond-body volume, the total circulating system volume, and the treatment volume used when any part of the system is isolated or bypassed.

1. Why True Water Volume Matters

Water volume is the denominator behind many pond calculations. A treatment rate expressed in milligrams per litre, grams per cubic metre, millilitres per 1,000 litres or parts per million becomes meaningless unless the number of litres is reasonably accurate. The same is true when a keeper says that a pond receives a 10 percent water change or turns over once every hour.

Aquaculture guidance frequently requires pond volume before a chemical amount can be calculated. University of Florida guidance, for example, determines chemical or salt quantities from the volume of water being treated. This is a fundamental principle: dose concentration is determined by the amount of chemical divided by the amount of water.

Accurate volume is especially useful for:

• Medication and parasite-treatment dosing.
• Salt additions and controlled salinity changes.
• Dechlorinator and water-conditioner dosing during major refills.
• Calculating the litres removed during a water change.
• Determining actual pond turnover from measured pump flow.
• Comparing stocking and feeding load with filtration capacity.
• Sizing heaters, chillers, ozone systems and some UV installations.
• Estimating emergency oxygen, chemical neutralization or replacement-water requirements.

The number does not need to be known to the nearest litre for a large koi pond. But a system recorded as 16,000 litres should not secretly be 11,000 or 22,000 litres. The larger the error, the less reliable every concentration-based decision becomes.

2. Understand the Three Different Volumes

Pond-body volume

Pond-body volume is the water physically inside the main koi pond at its normal operating water level. It excludes remote filter chambers, external tanks and most plumbing. This number is useful for design comparisons and for understanding how much swimming water the koi actually have.

Total circulating system volume

Total circulating system volume includes the pond plus the water that remains in operating filter chambers, settlement tanks, drum-filter troughs where applicable, biological chambers, pump chambers, connected quarantine or balance tanks if they are part of the same loop, and water-filled pipework. This is normally the most useful number for routine system calculations.

Treatment volume

Treatment volume is the volume that is actually exposed to a particular chemical. It may equal the total circulating volume, but not always. Some treatments require UV to be switched off while water continues through the filter. Other products may require carbon, zeolite, ozone, a shower, a protein skimmer or an entire filter loop to be bypassed. If a 2,000-litre filter system is isolated during treatment, the correct dosing volume may be the pond and remaining connected plumbing rather than the full system total.

This distinction prevents a common mistake: measuring the pond accurately once and then assuming that the same number applies to every future treatment configuration.

3. Use Litres and Cubic Metres as Your Primary Units

For Malaysian koi ponds, litres and cubic metres are the simplest units to record. One cubic metre equals exactly 1,000 litres for ordinary pond calculations. A pond containing 18.4 cubic metres therefore contains approximately 18,400 litres.

Be cautious when using gallons. A U.S. liquid gallon is approximately 3.785 litres, while an Imperial gallon is approximately 4.546 litres. The difference is about 20 percent. A treatment instruction that says only “per gallon” without identifying the gallon system should be clarified before use.

4. The Basic Rectangular Pond Formula

For a rectangular pond with vertical walls and a flat floor, the geometric calculation is straightforward:

Volume (m³) = length (m) × width (m) × water depth (m)

Volume (litres) = length × width × depth × 1,000

Measure the dimensions of the water, not the outside of the concrete structure. If a wall is 200 mm thick, external construction dimensions can significantly overstate the internal water volume. Measure to the finished waterproofed surfaces whenever possible.

Worked example: simple rectangular pond

A finished pond measures 4.5 m long, 2.5 m wide and the normal water depth is 1.50 m.

4.5 × 2.5 × 1.50 = 16.875 m³ = 16,875 litres.

If the pond normally runs at only 1.42 m of water because the overflow is lower than the coping, the correct geometric volume becomes 4.5 × 2.5 × 1.42 = 15.975 m³, or about 15,975 litres. A difference of only 8 cm in operating water level changed the calculation by 900 litres.

5. Measure Water Depth, Not Structural Depth

One of the most common errors is using excavation depth or wall height instead of actual water depth. A pond may be constructed 1.6 m deep but operate with 100–150 mm of freeboard below the coping. The bottom may also contain a raised screed, floor slope or drain sump that changes average depth.

For volume calculations, depth means the vertical distance from the normal water surface to the effective floor. If depth varies, use a method that reflects the changing cross-section instead of automatically using the deepest point.

6. Ponds with a Sloping Floor

A simple floor that slopes evenly from one depth to another can often be estimated using average depth. If the plan area remains essentially constant because the walls are vertical, average depth is the shallow depth plus the deep depth divided by two.

Average depth = (shallow depth + deep depth) ÷ 2

Worked example: evenly sloping rectangular floor

A pond is 5.0 m × 3.0 m. The floor slopes evenly from 1.2 m at one end to 1.6 m at the other.

Average depth = (1.2 + 1.6) ÷ 2 = 1.4 m.

Estimated volume = 5.0 × 3.0 × 1.4 = 21.0 m³ = 21,000 litres.

This shortcut works well when the slope is reasonably uniform. It becomes less reliable when the pond has multiple shelves, steep drain cones, irregular benches or curved transitions. In those cases, divide the pond into zones or use a metered fill.

7. Tapered Walls and Different Top and Bottom Areas

Some ponds are wider at the water surface than at the floor. Earthen ponds, landscaped ponds and some rendered concrete ponds have inward-sloping walls. Multiplying the top surface area by depth will overstate volume because the lower portion is smaller.

For a uniformly tapered shape where the upper and lower outlines are similar, a frustum approximation can be used:

V = h ÷ 3 × (A₁ + A₂ + √(A₁ × A₂))

Here, h is the vertical depth, A₁ is the top water-surface area, and A₂ is the bottom area. For more irregular profiles, measure several horizontal cross-sections and use a prismoidal or segmented method. In a residential koi pond, however, the practical answer is often simpler: use geometry for the design estimate, then record the actual metered fill after construction.

8. Circular Ponds

A vertical-sided circular pond is a cylinder. Use the internal water diameter, not the outer wall diameter.

Volume = π × radius² × water depth

Worked example: circular tank

A circular koi holding tank has an internal diameter of 3.0 m and a water depth of 1.2 m. Radius is 1.5 m.

π × 1.5² × 1.2 = approximately 8.48 m³, or 8,480 litres.

9. Oval and Elliptical Ponds

An oval pond with smooth elliptical sides can be estimated from the area of an ellipse:

Surface area ≈ π × (length ÷ 2) × (width ÷ 2)

Multiply that area by average water depth if the walls are close to vertical and the floor depth can reasonably be averaged.

Worked example: elliptical pond

An oval pond measures 4.0 m long, 2.5 m wide and averages 1.2 m deep.

Area ≈ π × 2.0 × 1.25 = 7.85 m². Volume ≈ 7.85 × 1.2 = 9.42 m³, or about 9,420 litres.

This is still an approximation. A natural-looking oval is rarely a perfect mathematical ellipse, so use a meter if dosing accuracy is important.

10. Irregular Ponds: Divide the Shape into Sections

For an L-shaped, kidney-shaped or highly sculpted pond, do not force the entire pond into one rectangle. Draw a plan view and divide it into smaller rectangles, triangles, semicircles or other shapes that can be calculated independently. Estimate the volume of each zone and add them together.

For a pond with changing depth, calculate each zone using its own average depth. A narrow shallow shelf may contain far less water than its visual footprint suggests, while a deep central trench may contribute more volume than expected.

When accuracy matters, use more segments rather than fewer. The purpose of segmentation is to make each part close enough to a simple geometric shape that the remaining error becomes small.

11. Account for Benches, Shelves, Steps and Raised Floors

Anything occupying space below the waterline displaces water. Concrete benches, plant shelves, internal steps, large permanent rock structures and thick raised floor sections reduce the true water volume compared with a simple bounding-box calculation.

The displacement can be estimated by calculating the submerged volume of the object and subtracting it from the pond estimate. For example, a solid bench 2.0 m long, 0.4 m wide and 0.3 m high displaces about 0.24 m³, or 240 litres, if fully submerged.

Do not waste time trying to calculate every rounded edge to the nearest litre. Focus on large features that can change the total by hundreds or thousands of litres.

12. Curved Corners and Rounded Transitions

Rounded corners are excellent for pond circulation and koi safety, but they also reduce volume compared with a perfect rectangular box. For a large pond, modest corner radii may create only a small percentage difference; for a compact pond, the effect can be meaningful.

If the pond is still at the design stage, computer-aided design software can calculate complex volumes directly from the final internal geometry. If the pond is already built, a water meter is usually easier and more accurate than reverse-engineering every radius.

13. Include Filter Chambers Correctly

A koi system can hold a substantial amount of water outside the pond. Traditional multi-chamber filters, settlement chambers, moving-bed biological tanks and pump chambers may together hold several cubic metres. Ignoring them can lead to underestimation of total circulating volume.

Measure each chamber at its normal operating water level. For a rectangular chamber, use internal length × internal width × operating water depth. If a chamber has a sloping base, use average depth or segment it.

Then consider displacement. Static media, brushes, mats, pipe manifolds and moving-bed media occupy some physical volume. A chamber that measures 1,000 litres geometrically does not necessarily contain 1,000 litres of water when filled with equipment. The exact displacement can be difficult to calculate, which is another reason why a full-system metered fill is preferable.

Bakki showers and trickle filters are different. Their boxes may have a large external volume, but while operating they usually contain only a thin film and transient water held around media rather than a full chamber of water. Do not count the entire physical box volume as circulating water.

14. Do Not Forget Pipework

Large koi systems may contain hundreds of litres in bottom-drain lines, skimmer lines, returns and filter interconnections. Pipe volume is calculated as a cylinder:

Pipe water volume = π × internal radius² × pipe length

Use the actual internal diameter where possible. A pipe sold as 110 mm or 4 inches may have a different internal diameter depending on wall thickness and standard.

Worked example: 110 mm internal-diameter pipe

If the actual internal diameter is 110 mm, the radius is 0.055 m. Twenty metres of water-filled pipe holds: π × 0.055² × 20 = approximately 0.190 m³, or 190 litres.

Pipework is often a small percentage of a residential pond, but on systems with long underground runs or several large gravity lines it is worth including.

15. The Best Practical Method: Meter the Initial Fill

Geometry tells you what the pond should hold. A metered fill tells you what the completed system actually accepted. For a new build, this is the best time to establish the definitive working volume.

Use a reliable water meter, hose-end totalizer, inline flow totalizer or the property water meter if no other water is being used during the fill. Record the initial reading before water enters the system and the final reading when the pond and all normal operating filter chambers have reached their usual running level.

If the meter records cubic metres, subtract the starting reading from the final reading and multiply by 1,000 to obtain litres.

Worked example: property water meter

Initial reading: 112.384 m³. Final reading after the complete pond and filter system reaches operating level: 128.962 m³.

128.962 − 112.384 = 16.578 m³ = 16,578 litres.

During this process, do not run washing machines, showers, irrigation or other significant water users on the same meter. If unavoidable, measure or account for that use separately.

16. Why a Water Meter Is Better Than Builder Estimates

A builder may quote a design volume before waterproofing thickness, benches, drain falls, chamber operating levels and final overflow height are known. The quoted figure may also be rounded for convenience. None of this necessarily means the builder did anything wrong; design volume and finished operating volume are simply different measurements.

Recording the actual fill removes most of those uncertainties at once. It automatically includes the final geometry, freeboard, filters and pipework—provided the entire system is filled in its normal configuration.

17. Flow Rate × Fill Time: Useful but Less Reliable

If a totalizing water meter is unavailable, you can measure the hose flow rate and multiply it by filling time. This can work, but flow may change as municipal pressure changes, other taps are opened, filters clog, hose positions change or storage tanks cycle.

Volume = average flow rate × time

Worked example

A hose averages 38 L/min and the measured filling time is 6 hours 30 minutes, or 390 minutes.

38 × 390 = 14,820 litres.

For better accuracy, measure the hose output into a calibrated container several times during the fill rather than once at the beginning. If the readings vary substantially, use the average and treat the result as an estimate.

18. Cross-Checking an Existing Pond with Salt

An established pond that cannot be drained can sometimes be checked by adding a precisely weighed amount of sodium chloride and measuring the resulting change in salinity after complete mixing. The principle is simple mass balance: a known mass of salt distributed through an unknown volume produces a measurable concentration change.

Volume (L) ≈ salt added (g) ÷ increase in salinity (g/L or ppt)

Because 1 ppt is equivalent to approximately 1 g/L for this practical freshwater calculation, the arithmetic is convenient. If using kilograms:

Volume (L) ≈ 1,000 × salt added (kg) ÷ salinity increase (ppt)

Worked example: salt cross-check

Baseline salinity is 0.12 ppt. Exactly 4.0 kg of fully dissolved sodium chloride is added. After sufficient circulation and mixing, the salinity reads 0.37 ppt.

Increase = 0.37 − 0.12 = 0.25 ppt.

Volume ≈ 1,000 × 4.0 ÷ 0.25 = 16,000 litres.

This method should be treated as a cross-check, not an excuse to add salt unnecessarily. The salinity meter must have adequate resolution and be calibrated or verified. Measure the baseline before adding anything, dissolve and distribute the salt fully, allow enough time for complete mixing, and take repeated readings from different parts of the system. Do not perform a water change between measurements. Avoid the method where additional salt is inappropriate for plants, other livestock, a current treatment protocol or the intended water chemistry.

19. Understand the Limits of Salinity and Conductivity Readings

A conductivity meter measures the ability of water to conduct electricity; it does not directly distinguish sodium chloride from all other dissolved ions. Some instruments convert conductivity into a displayed “salinity” using an internal relationship. That can be useful, but the reading may not behave exactly like a laboratory sodium-chloride concentration measurement in every freshwater composition.

For volume verification, use the same reliable instrument before and after the addition, follow the manufacturer’s calibration procedure, and make the concentration change large enough to be clearly distinguishable from measurement noise while remaining appropriate for the fish and system. If the meter only resolves to 0.1 ppt, trying to measure a 0.1 ppt change will produce a poor volume estimate.

20. Why “Salt Meter Volume Tests” Sometimes Give Strange Results

When a salt-derived estimate differs dramatically from geometry, investigate before accepting either number. Common causes include incomplete mixing, inaccurate salt weight, an uncalibrated meter, salt trapped temporarily in a filter or low-flow area, a background salinity that was not measured correctly, water replacement during the test, or a unit mistake between percent, ppt and ppm.

Remember the relationships: 1 percent is 10 ppt, and 1 ppt is 1,000 ppm. Confusing 0.1 percent with 0.1 ppt creates a tenfold error.

21. Medication Error: What a Wrong Volume Does

Suppose a treatment label calls for a certain amount per 1,000 litres. The owner believes the system is 20,000 litres and adds the dose for 20,000 litres, but the true treatment volume is only 15,000 litres.

The resulting concentration is 20,000 ÷ 15,000 = 1.33 times the intended concentration, or about 33 percent above target.

The reverse error is also important. If a true 20,000-litre system is treated as 15,000 litres, only 75 percent of the intended dose is delivered. An underdose may fail to control a parasite and can encourage repeated chemical exposure as the keeper tries again.

The safe response is not to “dose stronger just in case.” It is to improve the volume measurement, verify the diagnosis, follow the product label or professional treatment plan, and account for water chemistry and species sensitivity.

22. Converting Common Treatment Instructions

Once true treatment volume is known, dosing arithmetic becomes straightforward. Always confirm the label units and concentration of the exact product in hand.

Rate given in mL per 1,000 L

If the instruction is 25 mL per 1,000 L and treatment volume is 16,000 L: 25 × 16 = 400 mL.

Rate given in mg/L

If the target concentration is 2 mg/L in 16,000 L: 2 × 16,000 = 32,000 mg = 32 g of active ingredient. This calculation only determines the active-ingredient mass; a commercial product may contain a different percentage of active ingredient, so label directions still control product quantity.

Rate given in percent

One percent means one part per hundred. In water-treatment discussions, 0.1 percent salt is approximately 1 g/L, or 1 ppt. Keep percent, ppt, ppm and mg/L clearly separated in written records.

23. Calculate Water Changes from the Verified Volume

A true system volume also makes routine water changes measurable. If the circulating system contains 18,000 litres, a 10 percent replacement equals 1,800 litres and a 20 percent replacement equals 3,600 litres.

Do not assume that lowering the water level by 20 percent of pond depth always removes 20 percent of volume. That is true only when horizontal area stays constant with depth, as in a vertical-sided rectangular or cylindrical pond. In a pond with sloping walls, benches or irregular shelves, the upper 20 percent of depth may contain a different percentage of total water.

A permanently installed water meter on the refill line is an excellent long-term tool because it measures what was actually replaced rather than relying on a visual level mark.

24. Use True Volume to Calculate Turnover Correctly

Turnover is often quoted as though pump-label flow equals actual pond flow. It does not. A meaningful turnover calculation uses verified system volume and measured installed flow after head loss, pipe friction, valves, UV units and filter resistance are included.

Turnover time (hours) = system volume (L) ÷ actual flow (L/hour)

If a verified 18,000-litre system is actually moving 12,000 L/hour, theoretical turnover time is 1.5 hours. PE-007 and PE-008 discuss pump flow and turnover in detail; the important point here is that an inaccurate pond volume makes even a perfectly measured flow rate produce the wrong turnover figure.

25. Normal Operating Level Must Be Defined

Pond volume changes with water level. The difference may be modest in a small pond, but on a wide pond every centimetre represents substantial water. If a 30 m² pond surface drops 5 cm, volume falls by roughly 1.5 m³, or 1,500 litres, before considering shape changes.

Record the volume at a repeatable operating level—normally the level controlled by the overflow, autofill or preferred running height. If the pond routinely fluctuates significantly, note the approximate litres per centimetre near the operating level so treatment calculations can be adjusted when necessary.

26. Auto-Top-Up Systems Can Hide Water Loss

An automatic fill valve keeps the pond level stable but can conceal leaks, excessive filter-cleaning losses or evaporation. Installing a dedicated totalizing meter on the top-up line can provide two benefits: it helps verify initial pond volume during commissioning and later shows how much make-up water the system is consuming.

A sudden increase in daily top-up volume can become an early warning of a leak or malfunction long before the water level visibly drops.

27. Do Not Drain a Mature Pond Just to Measure It

If a mature, stable pond has never been accurately measured, a full drain is not automatically justified. Draining creates handling stress, disrupts temperature and water chemistry, may expose waterproofing or structural surfaces to conditions they normally do not experience, and can create avoidable risk for valuable koi.

Instead, improve the geometric estimate, measure filter and pipe volumes, inspect construction drawings if available, and use a carefully controlled cross-check. The next time the pond must legitimately be emptied for major repair or renovation, meter the refill and update the permanent record.

28. A Practical Accuracy Hierarchy

29. Commissioning Procedure for a New Koi Pond

The ideal time to establish pond volume is before koi are introduced. A simple commissioning procedure creates a permanent record that will be useful for the life of the pond.

1. Finish all waterproofing, pipework, filters and permanent internal structures before measuring.
2. Confirm that drain valves and waste lines are closed and that the system starts from a known empty or measured condition.
3. Record the water meter or totalizer reading.
4. Fill the pond gradually while checking for leaks and allowing connected gravity-fed chambers to equalize.
5. When pumps can be operated safely, run the system and allow all normal filter chambers and pipework to reach their operating levels.
6. Top up to the normal overflow or operating level.
7. Record the final meter reading and calculate total litres added.
8. If desired, separately calculate the pond-body geometry so you retain both pond-only and total-system figures.
9. Write the verified volume on the filter-house wall, in the pond log and in a digital record. Do not rely on memory.
10. Record the date, meter method, normal operating level and any unusual configuration so future owners or staff understand what the number represents.

30. Worked Example: From Design Estimate to Verified System Volume

Consider a koi pond with finished internal dimensions of 4.5 m × 2.5 m. The water level is 1.42 m above most of the floor, but the floor slopes toward bottom drains and includes rounded corners. The simple rectangular estimate gives 15,975 litres.

The filtration system includes a settlement/mechanical chamber, moving-bed chamber and pump chamber whose combined geometric water volume appears to be about 1,400 litres. Pipework is estimated at another 180 litres. A first planning estimate for the complete system is therefore around 17,555 litres before media displacement and curved geometry are considered.

During commissioning, however, a dedicated meter records 16,920 litres from empty to the normal fully circulating level. That measured figure should generally replace the rough 17,555-litre estimate for routine system calculations. The difference is plausible because benches, rounded transitions, filter media and internal plumbing displaced more water than the simple geometry allowed for.

The keeper can then record approximately 15,500–16,000 litres as pond-body water if needed for design discussion, and 16,920 litres as verified total circulating volume. A future treatment that bypasses a 1,200-litre filter loop would use a treatment-volume estimate closer to 15,700 litres, subject to the exact plumbing configuration.

31. Common Volume-Calculation Mistakes

• Using outside wall dimensions instead of finished internal water dimensions.
• Using construction depth instead of actual operating water depth.
• Using the deepest point as though the entire floor were equally deep.
• Ignoring inward-sloping walls, shelves, benches or large internal structures.
• Counting a shower filter as though its entire box were full of water.
• Ignoring several hundred litres in large filter chambers and gravity lines.
• Confusing U.S. gallons with Imperial gallons.
• Confusing percent, ppt, ppm, mg/L and g/L.
• Relying on a pump label to infer pond volume from fill time without measuring actual fill flow.
• Using a salinity method without measuring the baseline first.
• Reading a salt meter before the system has mixed completely.
• Using total system volume for a treatment when part of the system has been isolated.
• Rounding the figure aggressively upward because “more litres sounds better.”

32. How Accurate Does the Number Need to Be?

For broad construction planning, being within several percent is usually sufficient. For chemical treatment, closer is better. The required accuracy also depends on the treatment’s safety margin, the fish, water chemistry and the consequence of error.

A practical goal for a professionally built residential koi pond is to know the total circulating volume well enough that major treatment or water-change calculations are not being made from a guess. A metered fill often gets you much closer than elaborate geometry because it measures the completed system directly.

Do not create false precision. Writing “17,263.4 litres” does not make the number accurate if it came from rough tape measurements. Record an appropriate rounded working figure and note how it was obtained.

33. Keep the Volume Record Where It Can Be Found

The best measurement is useless if nobody can find it during an emergency. Record pond-body volume, total system volume and common treatment configurations in more than one place. A laminated card in the filter room, a note in the pond-maintenance log and a digital copy are inexpensive safeguards.

If the pond is modified—new filter chamber, raised water level, larger balance tank, converted shower, changed pipework or major structural alteration—update the volume record. The verified number belongs to a specific system configuration, not to the property forever.

34. Practical Pre-Treatment Checklist

• What is the verified total circulating volume?
• Was that volume measured by meter, geometry or another method?
• What is the current operating water level?
• Which filters, tanks and pipes will remain connected during treatment?
• Does the treatment label specify litres, U.S. gallons or Imperial gallons?
• Are the concentration units mg/L, ppm, ppt, percent, mL per 1,000 L or something else?
• Have product strength and active-ingredient concentration been checked?
• Does water chemistry change the recommended dose or safety margin?
• Will UV, ozone, carbon, zeolite or other equipment interfere with treatment?
• Is adequate aeration available for the treatment?
• If the diagnosis or dose is uncertain, has qualified fish-health advice been obtained before treating valuable koi?

35. The Most Useful Number Is the One You Can Defend

A koi pond does not need a laboratory-grade volumetric certification. It does need a realistic number based on a method that makes sense. For a new pond, meter the fill. For an existing pond, improve the geometry and cross-check it. For every treatment, think about which parts of the system are actually receiving the chemical.

Once the true volume is documented, many other pond-management decisions become easier. Water changes can be expressed in real litres. Turnover can be calculated from measured flow. Salt can be added deliberately rather than by guesswork. Treatment doses can follow the intended concentration instead of an uncertain pond label inherited from the builder.

In a high-value koi system, knowing water volume is one of the cheapest forms of risk control available.

Quick Reference Formulas

• Rectangular pond: L × W × average depth × 1,000 = litres.
• Circular pond: π × radius² × depth × 1,000 = litres.
• Elliptical pond: π × (L ÷ 2) × (W ÷ 2) × average depth × 1,000 = litres.
• Evenly sloping floor with vertical walls: average depth = (shallow + deep) ÷ 2.
• Pipe: π × internal radius² × length × 1,000 = litres.
• Water change: verified system litres × desired percentage = litres to replace.
• Turnover time: verified system litres ÷ measured L/hour = hours.
• Salt cross-check: volume (L) ≈ 1,000 × salt added (kg) ÷ salinity increase (ppt).