How the calculation works
Every shape works the same way: surface area × average depth = volume. That gives cubic feet, and one cubic foot of water is 7.48 US gallons — so multiply by 7.48 for gallons.
Surface area depends on the shape. A rectangle is length × width. A round pool is π × radius². An oval is π × (length ÷ 2) × (width ÷ 2). A kidney uses the pool-industry constant: (widest + narrowest) × length × 0.45. An L-shape is two rectangles added together.
Average depth on a sloping floor is simply the shallow end plus the deep end, divided by two.
Worked example
A 16 × 32 ft rectangular pool with a 3 ft shallow end and an 8 ft deep end. Average depth is (3 + 8) ÷ 2 = 5.5 ft. Surface area is 16 × 32 = 512 sq ft. Volume is 512 × 5.5 = 2,816 cubic feet, and 2,816 × 7.48 = about 21,060 gallons — roughly 79,700 litres, weighing close to 88 tons.
Assumptions
- Measurements are taken at the water line, not the top of the wall.
- The floor slopes evenly between the shallow and deep ends.
- Water weight uses 8.34 lbs per US gallon at normal pool temperatures.
- Kidney and freeform shapes use the industry-standard 0.45 area constant.
Limitations
- Steps, benches, swim-outs and tanning ledges all displace water — expect the true volume to be slightly lower than the figure shown.
- A floor with a sudden drop-off rather than an even slope will not average out the way this assumes.
- Freeform pools vary widely; for those, measuring in sections is more reliable than any single formula.
- Filling time ignores pressure drop, so a real fill usually takes a little longer.
Questions people ask
How many gallons is my pool?
Measure the length, width and depth at the water line, pick your shape above and the calculator does the rest. As a rough guide, a 16 × 32 ft pool at 5 ft average depth holds about 19,100 gallons, and a 24 ft round pool at 4 ft holds about 13,500.
Do I measure to the top of the wall or the water line?
The water line. Pools are filled to the middle of the skimmer opening, several inches below the top of the wall. Measuring to the wall overstates your volume, which leads straight to over-dosing chemicals.
What is average depth?
On a pool with a shallow end and a deep end, it is the two depths added together and divided by two. Switch to “Shallow & deep end” above and the calculator works it out for you.
How accurate is this?
For rectangles, round and oval pools it is exact geometry — the only error is in your tape measure. Kidney and freeform shapes use the standard 0.45 constant, which lands within roughly 5% for most designs. That is comfortably inside the tolerance of any chemical dose.
My pool is an odd shape. What do I do?
Break it into pieces. Most freeform pools are a rectangle plus a rounded end. Work out each piece separately, add the areas together, then choose “Custom” above and enter the total surface area.
Why does the volume matter so much?
Because every chemical dose is calculated per gallon. Get the volume wrong by a quarter and every dose is wrong by a quarter — which is how pools end up cloudy, over-chlorinated, or with alkalinity that will not sit still.
How long does a pool take to fill?
Enter your hose flow rate above. A typical garden hose runs 8–12 GPM, so a 20,000 gallon pool takes roughly 28 to 42 hours. To measure your own flow, time how long it takes to fill a 5-gallon bucket and divide 5 by the minutes taken.