The formulas, and why the number matters
Pool volume is just surface area multiplied by average depth, which for a sloped floor is the shallow-end depth plus the deep-end depth divided by two. For a rectangular pool the area is length × width. For a round pool it is π × (diameter ÷ 2)². For an oval, it is π × (length ÷ 2) × (width ÷ 2). That gives cubic feet; multiply by 7.48 to get US gallons, and multiply gallons by 3.785 for litres. Measure the water dimensions rather than the outside of the coping, and if your pool has a sharp hopper or a shallow tanning ledge, treat it as two sections and add them together for a closer figure.
Getting this right matters because volume is the basis of every chemical decision you make. Chlorine, algaecide, stabiliser, pH increaser and shock are all dosed per unit of water, so a volume estimate that is 20% off means every dose you ever add is 20% off in the same direction. Under-dose and the pool will not hold a residual; over-dose and you risk cloudy water, corroded fittings, bleached liners and irritated skin and eyes. Volume also sets your turnover rate — the time your pump needs to circulate the whole pool — which drives pump and filter sizing and how many hours a day you should run the system.
Work the number out once, write it on the inside of the equipment cabinet door, and use the same figure every season. Dosing instructions are almost always written per 10,000 gallons, so knowing your pool is, say, 17,000 gallons turns every product label into simple arithmetic.
Volume figures are estimates for planning only. Always dose chemicals according to the product label and your own test results, add them one at a time, and never mix pool chemicals together in a container.