Pool & spa
Pool Gallon Calculator
Converts pool dimensions to gallons, the unit every chemical label uses.
Result
Figures are US gallons unless the imperial line is used. Water weighs 8.34 pounds per US gallon, which adds up quickly on even a modest pool.
Get a Free Business AuditHow the calculation works
Cubic feet times 7.48052 gives US gallons. The same geometry applies as for volume: length times width times average depth for a rectangle, pi times radius squared times average depth for a round pool, with average depth being the mean of the ends on an evenly sloping floor. Gallons is the working unit because chemical dosing instructions are written per ten thousand gallons, so converting once means every subsequent dose calculation is straightforward.
What moves the number
- Average depth, the usual source of error.
- Shape, with freeform pools needing sectioning.
- Whether the measurement is US or imperial gallons, which differ.
- Whether an attached spa is included or dosed separately.
Where this one goes wrong
US and imperial gallons are not the same, and chemical products sold in the United States are dosed per US gallon, so a figure worked out in imperial gallons will under dose by roughly a fifth. If you are working from a source using imperial measures, convert. An attached spa is the other thing to settle: if it shares water with the pool through a spillover it is part of the same volume, and if it isolates it needs dosing separately, and treating a combined system as pool-only under doses the whole thing.
Before you quote
- Confirm the figure is US gallons if using US chemical dosing.
- Decide whether an attached spa shares the water volume.
- Use 7.48 cubic feet to gallons rather than a remembered approximation.
Where the rules come from
Questions
About this calculator
How many gallons in a cubic foot of water?
About 7.48 US gallons. That conversion is the whole calculation once you have the volume in cubic feet. Imperial gallons are larger, so a figure worked out in imperial will under dose chemicals sold against US gallon instructions by roughly a fifth.
Should the spa be included in the pool volume?
If it shares water with the pool through a spillover, yes, it is part of the same body of water and dosing has to cover both. If it isolates with a valve and is treated separately, it needs its own volume figure. Treating a combined system as pool-only under doses everything.
Why are chemical doses given per ten thousand gallons?
It is simply the industry convention, which makes dosing arithmetic easy once you know your volume. A twenty thousand gallon pool takes twice the stated dose, a fifteen thousand gallon pool one and a half times. Knowing your gallons turns every product label into a direct instruction.
The formula
gallons = length x width x average depth x 7.48
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How to use this calculator
- Pool shape. Enter your own figure; the page starts at Rectangular.
- Length (ft) (ft). Enter your own figure; the page starts at 32 ft.
- Width (ft) (ft). Enter your own figure; the page starts at 16 ft.
- Diameter if round (ft) (ft). Enter your own figure; the page starts at 21 ft.
- Shallow end depth (ft) (ft). Enter your own figure; the page starts at 3 ft.
- Deep end depth (ft) (ft). Enter your own figure; the page starts at 6 ft.
The result updates as you type, so you can try a figure, see what it does to gallons, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Pool shape | Rectangular |
| Length (ft) | 32 ft |
| Width (ft) | 16 ft |
| Diameter if round (ft) | 21 ft |
| Shallow end depth (ft) | 3 ft |
| Deep end depth (ft) | 6 ft |
What comes out
| Result | Value |
|---|---|
| Gallons main result | 17,234 |
| Litres | 65,230 |
| Imperial gallons | 14,351 |
| Weight of the water | 143,731 |
17,234 gallons, which weighs 143,731 pounds. That weight is why pool walls and decks are engineered rather than guessed at.
How sensitive the answer is
Holding everything else steady and changing only length (ft), this is what happens to gallons. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Length (ft) (ft) | Gallons |
|---|---|
| 16 ft | 8,617 |
| 24 ft | 12,925 |
| 32 ft example above | 17,234 |
| 48 ft | 25,851 |
| 64 ft | 34,468 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Pool shape
- Choose from Rectangular, Round, Oval.
- Length (ft) (ft)
- Measured in ft, starting at 32 ft.
- Width (ft) (ft)
- Measured in ft, starting at 16 ft.
- Diameter if round (ft) (ft)
- Measured in ft, starting at 21 ft.
- Shallow end depth (ft) (ft)
- Measured in ft, between 0 and 20, starting at 3 ft.
- Deep end depth (ft) (ft)
- Measured in ft, between 0 and 40, starting at 6 ft.
What the results tell you
| Result | What it is |
|---|---|
| Gallons | The headline result. A number. |
| Litres | A number. |
| Imperial gallons | A number. |
| Weight of the water | A number. |
Keep going
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