Quick answer: for a rectangular pool, gallons = length × width × average depth × 7.48 (feet). Round and oval pools use × 5.9 instead, and kidney pools use (width A + width B) × length × average depth × 3.38. Average depth on a sloped floor is simply the shallow and deep depths added and halved.
The formulas, and where those odd multipliers come from
Pool professionals work with a set of shortcut multipliers. They are not arbitrary — each one is a shape constant with the gallon conversion already folded in:
| Pool shape | Gallons (feet) | Where the number comes from |
|---|---|---|
| Rectangular | L × W × D × 7.48 | 7.4805 US gallons per cubic foot |
| Round | D² × depth × 5.9 | π/4 (0.7854) × 7.4805 = 5.875 |
| Oval | L × W × depth × 5.9 | Ellipse area is also π/4 × L × W |
| Kidney | (A + B) × L × depth × 3.38 | 0.45 shape factor × 7.4805 = 3.37 |
The calculator above uses the exact constants rather than the rounded trade versions, so its answers run a fraction of a percent below the shortcut figures. Either is fine for pool chemistry; the exact version is simply the one you can check with a pencil.
Why averaging the depths works
A pool floor that slopes evenly from a 3 ft shallow end to an 8 ft deep end is a wedge sitting under a rectangular box. The wedge holds exactly what a flat floor at the mean depth of 5.5 ft would hold — the extra water at the deep end precisely balances what is missing at the shallow end. That is why the average works, and it is exact rather than an approximation, provided the slope is constant.
It stops being exact when a pool has a hopper bottom — a flat shallow section, then a sharp drop, then a flat deep section. For those, split the pool into sections, calculate each, and add. A common approximation for hopper pools is to use (shallow + deep) ÷ 2 anyway and accept a few percent of error, which is usually acceptable for dosing.
Standard pool sizes and their gallons
Useful for a sanity check on your own measurement:
| Pool | Size | Avg depth | US gallons |
|---|---|---|---|
| Above-ground round | 15 ft ⌀ | 4 ft | ≈ 5,300 |
| Above-ground round | 18 ft ⌀ | 4 ft | ≈ 7,600 |
| Above-ground round | 21 ft ⌀ | 4 ft | ≈ 10,400 |
| Above-ground round | 24 ft ⌀ | 4 ft | ≈ 13,500 |
| Above-ground round | 27 ft ⌀ | 4 ft | ≈ 17,100 |
| Above-ground oval | 15 × 30 ft | 4 ft | ≈ 10,600 |
| In-ground rectangular | 12 × 24 ft | 5 ft | ≈ 10,800 |
| In-ground rectangular | 14 × 28 ft | 5.5 ft | ≈ 16,100 |
| In-ground rectangular | 16 × 32 ft | 5.5 ft | ≈ 21,100 |
| In-ground rectangular | 20 × 40 ft | 6 ft | ≈ 35,900 |
| Lap pool | 10 × 50 ft | 4 ft | ≈ 15,000 |
What your volume number is actually for
Nearly every pool chemical is dosed per 10,000 gallons. Here is what a standard adjustment looks like — and why a bad volume estimate is expensive:
| Goal | Product | Dose per 10,000 gal |
|---|---|---|
| Raise free chlorine by 1 ppm | Liquid chlorine (12.5%) | ≈ 10.7 fl oz |
| Raise free chlorine by 1 ppm | Cal-hypo (73%) | ≈ 1.8 oz by weight |
| Shock to 10 ppm | Liquid chlorine (12.5%) | ≈ 1 gallon |
| Raise pH | Soda ash | ≈ 6 oz per 0.2 pH |
| Lower pH | Muriatic acid (31%) | ≈ 12 fl oz per 0.2 pH |
| Raise total alkalinity by 10 ppm | Baking soda | ≈ 1.5 lb |
| Raise calcium hardness by 10 ppm | Calcium chloride | ≈ 1.25 lb |
| Raise stabiliser by 10 ppm | Cyanuric acid | ≈ 13 oz |
These are typical figures for planning; always follow the dose printed on your product, since concentrations vary between brands. Add chemicals one at a time, run the pump, and retest before adding anything else — and never mix concentrated chemicals together.
The arithmetic that matters: if your pool actually holds 21,000 gallons but you have been dosing for 25,000, every addition is 19% too strong. Over a season that is real money in wasted product, a faded liner, and water that is regularly out of balance in ways that are hard to diagnose.
Three numbers that come out of this one
Turnover rate and pump sizing
Pool water should circulate completely every 8 to 12 hours. A 21,000-gallon pool on an 8-hour turnover needs 21,000 ÷ 8 ÷ 60 ≈ 44 gallons per minute of flow. Buy a pump before you know your volume and you are guessing at the single most important spec.
Heater sizing and run time
Raising a pool by one degree Fahrenheit takes about 8.34 BTU per gallon. Warming a 21,000-gallon pool by 10 °F therefore needs roughly 1.75 million BTU — about 8 hours of run time from a 250,000 BTU/hr heater, before accounting for surface heat loss. A cover changes this dramatically, since most loss is evaporation from the surface.
Filling cost and time
A garden hose delivers roughly 8 to 12 gallons per minute. Filling that same pool takes about 30 to 44 hours of continuous running, and at a typical municipal rate of $4 per thousand gallons it costs somewhere near $85 in water alone. Worth knowing before you drain a pool casually.
Common mistakes & pro tips
- Measuring the wall height instead of the water depth. The biggest error on above-ground pools. A 52-inch wall usually holds about 48 inches of water — using 52 inflates the volume by roughly 8%.
- Measuring outside the coping. Measure at the waterline, inside the pool surface.
- Using the deep end as the depth. Always average shallow and deep. Using the deep end alone on a 3 ft to 8 ft pool overstates volume by 45%.
- Trusting the builder's round number. "About 20,000 gallons" is a sales figure. Measure it once yourself and write it on the equipment pad in permanent marker.
- Pro tip — verify with the water meter. When topping up, read your meter before and after. If the pool needed 500 gallons to rise one inch, and your surface area is 512 ft², check: 512 × (1/12) × 7.48 ≈ 319 gallons per inch. A large mismatch means your dimensions are off — or you have a leak.
- Pro tip — irregular free-form pools. Break the shape into rectangles and half-circles, calculate each, add them up. Or measure the surface area from a satellite image and multiply by the average depth and 7.48.
How to use this calculator
- Pick your pool shape. Rectangular, round, oval or kidney. Measure at the waterline, not the outside of the coping.
- Measure length and width. For a kidney pool, measure the two widest widths across the pool and add them together.
- Enter the depths. For a uniform-depth pool enter only the shallow depth. For a sloped floor add the deep-end depth too, and the calculator averages them.
- Read the gallons. The result appears in US gallons by default, with liters and cubic units alongside. Use this figure for every chemical dose.
Frequently asked questions
How do I calculate how many gallons my pool holds?
For a rectangular pool, multiply length × width × average depth to get cubic feet, then multiply by 7.48 for US gallons. A 16 × 32 ft pool averaging 5.5 ft deep holds 2,816 ft³ ≈ 21,064 gallons. Round and oval pools use length × width × average depth × 5.9, and kidney pools use (width A + width B) × length × average depth × 3.38.
How do I find average depth for a sloped pool?
Add the shallow-end depth to the deep-end depth and divide by two. A pool that runs 3 ft at the shallow end and 8 ft at the deep end averages 5.5 ft. This works because a constant slope makes the floor a wedge, and a wedge holds exactly the same as a flat floor at the mean depth.
How many gallons is a 24 foot round pool?
A 24 ft round above-ground pool with a 52-inch wall, filled to about 48 inches (4 ft), holds roughly 13,500 US gallons. At 52 inches of actual water it would be about 14,700 gallons — always calculate from the water depth, not the wall height.
Should I measure to the top of the wall or the waterline?
The waterline. Pools are never filled to the rim — typically the water sits mid-skimmer, about 4 to 6 inches below the top. Using the wall height instead of the water depth overstates a 24 ft round pool by well over a thousand gallons, which means overdosing every chemical you add.
Why does pool volume matter for chemicals?
Every dose on a chemical label is specified per 10,000 gallons. If your real volume is 21,000 gallons and you assumed 25,000, you overdose by 19% on every single addition — which wastes money, can bleach a liner and can push chlorine to levels that make the water unsafe to swim in.
How much does a pool of water weigh?
Water weighs 8.34 pounds per US gallon, so a 20,000-gallon pool holds about 166,800 lb — roughly 83 tons. That is why deck and structural design matters, and why draining an in-ground pool without professional advice risks it floating out of the ground.