Pool Volume Calculator

Background.

How many gallons is my pool? It is the first question behind almost every pool-care job, because chlorine, shock, pH adjusters, stabiliser and salt are all dosed per unit of water, and most US product labels write their directions per 10,000 gallons. A wrong volume makes every dose wrong by the same proportion. An owner who assumes 15,000 gallons for a pool that really holds 18,514 under-doses everything by about 19%, then wonders why the chlorine never seems to hold.

Few pools come with the figure written down, but any of them can be measured with a tape. Volume is surface area times average depth. A rectangle's area is length times width; a round pool is a circle, π times the radius squared; an oval is treated as an ellipse, π times half the length times half the width; and a kidney or freeform outline is taken as 85% of the rectangle that would just enclose it. Average depth is the midpoint of the shallow and deep ends, which is exactly right when the floor slopes evenly from one end to the other and increasingly wrong the further it departs from that.

You can measure in feet or metres, and the answer arrives in US gallons, litres, cubic feet and cubic metres together, so it matches whichever unit your chemical label, heater brochure or pump chart happens to use.

The figure is geometric. It counts the water inside the outline of the shell, not what sits in pipes, the filter or a surge tank, and it does not subtract the water displaced by steps, benches and sun ledges. For a backyard pool those differences are usually smaller than the error in measuring depth. Where you need better than that — a public pool being commissioned, or a freeform shape the 85% rule does not describe — read the water meter before and after a fill. The difference between the two readings is the volume, no geometry required.

What is pool volume calculator?

Pool volume is the quantity of water a pool holds at its normal operating level: measured to the waterline, not to the top of the wall. In the United States it is quoted in US liquid gallons, each defined as 231 cubic inches (about 3.785 litres); most other countries use litres or cubic metres, with one cubic metre equal to 1,000 litres. It is the denominator in nearly every number a pool owner tests for. Free chlorine, cyanuric acid, salt, alkalinity and calcium hardness are all concentrations in parts per million, and how much product it takes to move any of them depends directly on how much water there is to dilute it in.

How to use this calculator.

1. 01Choose the shape. Kidney / irregular covers freeform outlines; if your above-ground oval has two straight sides with rounded ends, read the FAQ on that shape first, because the Oval option assumes a true ellipse.
2. 02Set the unit system before you type dimensions: feet in U.S. customary mode, metres in metric mode. All four volume units are reported either way.
3. 03Measure length and width at the waterline, inside wall to inside wall. For a kidney or freeform pool use the longest length and the widest width. For a round pool enter the diameter instead; length and width are ignored for that shape, and diameter is ignored for the others. Soft-sided and older round pools are seldom perfect circles, so measure through the centre twice at right angles and use the average.
4. 04Measure depth from the water surface down to the floor at the shallowest and the deepest points. The deep figure must be at least as large as the shallow one; for a flat floor or an above-ground pool, enter the same water depth in both boxes.
5. 05Use gallons or litres with chemical labels, and cubic feet or cubic metres where a heater or pump sizing sheet asks for them.

The formula.

Volume = plan area × average depth, with average depth = (d₁ + d₂) ÷ 2, where d₁ is the shallow depth and d₂ the deep depth. The plan area depends on the shape you pick. A rectangle is L × W. A round pool of diameter D is π × (D ÷ 2)². An oval is π × (L ÷ 2) × (W ÷ 2), the area of an ellipse, which covers π/4 — about 78.5% — of the rectangle around it. A kidney or irregular pool is L × W × 0.85, a trade rule of thumb that assumes the outline fills 85% of its bounding rectangle.

With dimensions in feet the product is in cubic feet. Gallons follow at 7.48051948 US gallons per cubic foot, which is 1,728 cubic inches divided by the 231 cubic inches in a gallon. Cubic metres follow at 0.028316846592 per cubic foot, an exact figure because the foot is defined as 0.3048 m, and litres are cubic metres × 1,000. In metric mode your metre measurements are first converted to feet (divided by 0.3048) and the same chain runs, so a 10 m by 5 m pool averaging 1.5 m deep still comes out at 75 cubic metres, 75,000 litres.

Averaging the two end depths is exact only when the floor is a straight ramp from shallow to deep, or flat. Many in-ground pools are not built that way: a long, level shallow area followed by a short, steep hopper holds much less water than the midpoint depth suggests, because most of the surface sits over the shallow floor. In that case measure the pool in sections, as one of the questions below shows.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Centers for Disease Control and Prevention (2025). About the MAHC Current Edition. CDC Model Aquatic Health Code.
2. [2]Centers for Disease Control and Prevention (2024). Model Aquatic Health Code, 5th edition. CDC.
3. [3]National Institute of Standards and Technology (2026). NIST Handbook 133, Appendix E: General Tables of Units of Measurement. U.S. Department of Commerce.
4. [4]National Institute of Standards and Technology (2008). Guide for the Use of the International System of Units (SI), NIST Special Publication 811. U.S. Department of Commerce.
5. [5]Bureau International des Poids et Mesures (2019). The International System of Units (SI Brochure), 9th edition. BIPM.