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How To Calculate A Pool Volume
Pool volume is the number every other pool calculation depends on, because chemicals, heating and pump run times are all dosed per litre rather than per pool.
Quick Answer
volume = length x width x average depth
- V
- Volume of water, in cubic metres
- L
- Length of the pool
- W
- Width of the pool
- d
- Average water depth
Multiply length by width by the average depth. For a pool that slopes, average the shallow and deep ends first. An eight by four metre pool running from 1.2 to 1.8 metres holds 48 cubic metres, which is 48000 litres.
What Is A Pool Volume?
The volume of a pool is its length multiplied by its width multiplied by its average depth. An eight by four metre pool running from 1.2 to 1.8 metres deep holds forty eight cubic metres, which is forty eight thousand litres.
The average depth is what makes a sloped pool tractable. If one end is 1.2 metres and the other is 1.8, the average is 1.5, and the pool holds the same water as a flat pool 1.5 metres deep throughout. This is exact for a pool whose floor slopes evenly, which most do.
A cubic metre is a thousand litres, which makes the metric conversion trivial. Forty eight cubic metres is forty eight thousand litres. In imperial units the same pool holds about twelve thousand six hundred and eighty US gallons.
Gallons are worth converting carefully, because US and imperial gallons differ by about twenty percent. A US gallon is about 3.785 litres and an imperial gallon about 4.546. Pool chemicals sold in one market are dosed for that market's gallon.
The volume is what every other pool calculation starts from. Chlorine doses are quoted in grams per thousand litres, shock treatments per ten thousand litres, and algaecide per litre. Without the volume you cannot dose anything correctly.
Heating calculations use the same number. The energy needed to raise the water temperature is the mass of water times its specific heat times the temperature rise, and the mass comes straight from the volume at one kilogram per litre.
Filter run time scales with volume too. A pump rated in litres per hour needs to turn the whole pool over once or twice a day, so the run time is the volume divided by the pump rate multiplied by the number of turnovers wanted.
Round pools use the cylinder formula instead: pi times the radius squared times the average depth. An oval pool is treated as a rectangle with a correction factor, usually around 0.89, because it is a rectangle with the corners rounded off.
Measure the water depth, not the pool wall. A pool filled to ten centimetres below the coping holds noticeably less than its structural depth suggests, and for a small pool that difference can be several percent of the volume.
Irregular pools with steps, benches or a separate spa section should be split into simple shapes, calculated separately and added. The same discipline as splitting an L shaped room into rectangles applies, and the trap of double counting the overlap is the same too.
Finally, always round sensibly. A pool volume computed to four decimal places implies a precision no tape measure provides. Round to the nearest cubic metre, or the nearest hundred litres, and dose from that.
Formula
V = length x width x average depth
Average the shallow and deep ends first if the floor slopes. The result is in cubic metres.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| V | Volume | cubic metres | Water the pool holds. |
| L | Length | metres | Long side of the pool. |
| W | Width | metres | Short side of the pool. |
| d | Average depth | metres | Mean of the shallow and deep water depths. |
V = pi x radius^2 x average depth
A cylinder. Halve the diameter to get the radius before squaring it.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| V | Volume | cubic metres | Water the pool holds. |
| r | Radius | metres | Half the pool's diameter. |
| d | Average depth | metres | Depth of the water. |
litres = cubic metres x 1000
One cubic metre is exactly a thousand litres. US gallons are about 264.17 per cubic metre.
| Symbol | Meaning | Unit | Notes |
|---|---|---|---|
| litres | Litres | litres | Metric volume, used for chemical dosing. |
| gallons | US gallons | gallons | Imperial volume, about 264.17 per cubic metre. |
How To Calculate A Pool Volume
- 1
Measure the length and width of the water
Measure at the waterline rather than the coping, and measure the water surface rather than the pool shell. Steps and ledges are better handled as separate shapes.
- 2
Measure both the shallow and deep water depths
Take the depth to the water surface, not to the top of the wall. A pool filled below the coping holds less than its structural depth implies.
- 3
Average the two depths
Add them and divide by two. Depths of 1.2 and 1.8 give an average of 1.5, which is the depth of the flat pool holding the same water.
- 4
Multiply length by width by average depth
Eight times four times 1.5 is forty eight cubic metres. There is no correction factor for a rectangular pool with an evenly sloping floor.
- 5
Convert to litres and check against the label on your chemicals
Multiply cubic metres by a thousand for litres, or by 264.17 for US gallons. Chemical doses are quoted per thousand or per ten thousand litres, so the conversion has to be right.
Examples
Example 1: An eight by four metre pool from 1.2 to 1.8 deep
- Length
- 8 m
- Width
- 4 m
- Shallow
- 1.2 m
- Deep
- 1.8 m
| Step | Calculation | Result |
|---|---|---|
| Average depth | (1.2 + 1.8) / 2 | 1.5 |
| Volume in gallons | 48 x 264.17 | 12680.26 |
| Volume | 8 x 4 x 1.5 | 48 |
Result: The pool holds 48 cubic metres, which is 48000 litres, or about 12680 US gallons.
Example 2: A ten by five metre pool from 1 to 2 metres
- Length
- 10 m
- Width
- 5 m
- Shallow
- 1 m
- Deep
- 2 m
| Step | Calculation | Result |
|---|---|---|
| Average depth | (1 + 2) / 2 | 1.5 |
| Volume in litres | 75 x 1000 | 75000 |
| Volume | 10 x 5 x 1.5 | 75 |
Result: A ten by five metre pool with an average depth of 1.5 holds 75 cubic metres, which is 75000 litres.
Example 3: A small six by three metre plunge pool
- Length
- 6 m
- Width
- 3 m
- Shallow
- 1.1 m
- Deep
- 1.6 m
| Step | Calculation | Result |
|---|---|---|
| Average depth | (1.1 + 1.6) / 2 | 1.35 |
| Volume in litres | 24.3 x 1000 | 24300 |
| Volume | 6 x 3 x 1.35 | 24.3 |
Result: The plunge pool holds 24.3 cubic metres, which is 24300 litres, and a dose of one gram per thousand litres needs about 24 grams.
Calculator
Volume in cubic metres
48
- Volume in litres
- 48,000
- Volume in US gallons
- 12,680.2585
- Average depth
- 1.5
Values update as you type. This calculator covers the single scenario its formula assumes — see Common Mistakes for what it leaves out.
Prefer a full-width tool? Open the A Pool Volume calculator page.
Common Mistakes
Using the structural depth instead of the water depth
The pool is never filled to the very top. Measuring to the coping rather than the waterline overstates the volume, and every chemical dose built on it will be too high.
Forgetting to average a sloped floor
Multiplying by the shallow depth understates the volume badly, and multiplying by the deep depth overstates it. The average of the two is exact for an evenly sloping floor.
Confusing US and imperial gallons
The two differ by about twenty percent. Dosing a pool using the wrong gallon definition can mean a significant over or under dose of chemicals.
Treating an oval pool as a rectangle
An oval is a rectangle with rounded corners, so it holds less. A correction factor of about 0.89 is the usual approximation.
Squaring the diameter for a round pool
The cylinder formula needs the radius. Using the diameter quadruples the answer, the same trap as every other circular calculation.
Ignoring steps and benches
Steps, ledges and built in spas displace water. For a pool with generous entry steps, leaving them out can overstate the volume by several percent.
Quoting the volume to unrealistic precision
A tape measure does not justify four decimal places. Round to the nearest cubic metre and dose from that, which is well within the tolerance of any chemical treatment.
FAQ
How do you calculate the volume of a pool?
Multiply the length by the width by the average depth. For a pool that slopes, average the shallow and deep ends first, then multiply.
How many litres are in a cubic metre?
Exactly one thousand. So a pool of 48 cubic metres holds 48000 litres, which is the figure chemical doses are based on.
How do I work out the average depth?
Add the shallow and deep water depths and divide by two. Depths of 1.2 and 1.8 metres give an average of 1.5.
How many gallons does my pool hold?
Multiply the cubic metres by 264.17 for US gallons, or by 219.97 for imperial gallons. The two differ enough that it matters for dosing.
Does a round pool use a different formula?
Yes. A round pool is a cylinder, so the volume is pi times the radius squared times the average depth. Use the radius, not the diameter.
Why does the pool volume matter so much?
Because every chemical dose, heater sizing and filter run time is quoted per litre or per thousand litres. Getting the volume wrong scales every one of those errors.
References
- [1]Wikipedia, Swimming pool — https://en.wikipedia.org/wiki/Swimming_pool
- [2]Wikipedia, Cubic metre — https://en.wikipedia.org/wiki/Cubic_metre
- [3]Wikipedia, Gallon — https://en.wikipedia.org/wiki/Gallon
- [4]Wikipedia, Volume — https://en.wikipedia.org/wiki/Volume