How much water is actually in the pool?
Pick the shape, enter the surface dimensions and the shallow and deep water depths, and this calculator works out the volume from the pool’s average depth, the correct method for the standard linear slope, and reports it in US gallons, litres and cubic metres, with imperial gallons alongside for anyone whose dosing chart speaks that language. Enter water depths, not wall depths: the empty gap at the top, the freeboard, holds no water and doses no chemicals.
Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. Chemical dosing follows your product labels and water testing, with this volume as the input, never the instruction.
What this calculator does
Pool volume drives everything in pool care: chemical doses, salt for a chlorine generator, heater and pump sizing, and the water bill for a refill. It is also routinely overstated, because the tempting method (surface area times the deep-end depth) prices the whole pool as if it were the deep end. The honest method for the standard pool floor, which slopes evenly from shallow to deep, is the average of the two water depths, and that is what runs here. Shapes beyond the rectangle use their proper geometry: the ellipse for ovals, and for kidney and freeform pools the trade’s standard approximation, which the page names as an approximation rather than dressing up as geometry.
How to use it
- Measure water depth, not wall depth. Drop a tape from the waterline to the floor at both ends. A pool built “8 feet deep” usually holds about 7.5 feet of water under a 6-inch freeboard, and dosing for 8 buys chemicals for air.
- Measure the waterline dimensions too. Coping overhangs and tapered walls make the surface at the waterline the honest outline; for ovals, length and width at the waterline feed the ellipse.
- Kidney pools: measure both lobes. The two widths across the wide and narrow ends, plus the full length, are what the approximation consumes. A tape across the middle measures neither lobe.
- Use the gallon figure your products use. US chemicals dose in US gallons; the imperial figure differs by about 20 percent, the same Atlantic gap as every other gallon on this site, and the results print both so the right one can be picked consciously.
The formulas
- Average depth = (shallow + deep) / 2, valid when the floor slopes evenly between them, which standard pool floors do.
- Rectangle: volume = length × width × average depth. Round: π × radius² × average depth. Oval: π × (length/2) × (width/2) × average depth.
- Kidney / freeform: volume ≈ 0.45 × (lobe A + lobe B) × length × average depth, the pool trade’s standard approximation for the shape.
- Conversions: 1 cu ft = 7.48052 US gallons = 28.3168466 litres; 1 m³ = 1,000 litres = 264.172 US gallons; imperial gallons = litres / 4.54609.
Worked examples
The standard backyard pool. A 16 × 32 ft rectangle, 3 ft at the shallow end and 8 ft at the deep: average depth 5.5 ft, volume 16 × 32 × 5.5 = 2,816 cu ft, which is 21,065.14 US gallons (79,740.24 litres, 79.74 m³). Priced at the deep-end depth instead, the same pool “holds” over 30,600 gallons: the 45 percent overdose, in chemicals, that the averaging step exists to prevent.
The kidney. Length 30 ft, lobes of 14 and 10 ft, average depth 5 ft: 0.45 × 24 × 30 × 5 = 1,620 cu ft, about 12,118.44 US gallons. Approximate, as labelled, but a far better dosing basis than either a rectangle drawn around the outline (which would overstate it by a third) or a shrug.
Reading your results
Read the volume against the pool’s advertised size if it has one: builders quote nominal volumes that often use wall depths and outside dimensions, so a result 10 to 20 percent lower is the calculator being honest, not wrong. Read the four units as one number in four dialects and pick by the label in your hand. And read a suspiciously round result as a prompt to re-measure the depths, since those two fields, entered from memory more than any others, swing the answer most.
Limitations
- Average depth assumes a linear slope. Pools with a sudden drop-off, a sun shelf, benches or a beach entry hold less than the two-depth average suggests; a shelf’s volume can be subtracted as its own shallow box with the area and volume calculator.
- Freeform shapes are approximated. The 0.45 kidney factor is the trade standard, not a survey. For a refill quote or a liner order, the manufacturer’s figure for the exact model outranks it.
- Steps, spas and water features are separate volumes. An attached spa is its own small pool with its own depths; add its volume separately rather than stretching the main shape to cover it.
- Volume is the input to dosing, not the dose. Chemical amounts depend on the product concentration and the water’s measured state; labels and test kits decide them, with this number in hand.
Frequently asked questions
Why average the two depths instead of using the deep end?
Because the floor slopes: for every foot of water column near the deep end there is a matching short column near the shallow end, and the pair together hold exactly what a uniform pool at the average depth holds, as long as the slope is even. Using the deep depth throughout prices the shallow half as deep water, which on a typical pool overstates the volume by a third to a half, and every chemical dosed by volume inherits the error.
How do I volume a pool with a diving well or hopper bottom?
In sections. A hopper bottom (flat deep area with sloped sides feeding it) is not a linear slope end to end, so split the pool at the break-over point: the sloped section by this calculator’s average, the flat deep section as surface area times its true depth, and add. The area and volume calculator linked below handles the box arithmetic; the discipline is the same one this page teaches, applied twice.
US or imperial gallons: does it matter?
By about 20 percent on every dose, yes. An imperial gallon is 4.54609 litres; a US gallon is 3.785 litres. US pool products, pumps and salt systems are specified in US gallons; older British and some Commonwealth references use imperial. The results print both, labelled, because the expensive version of this mistake is a salt cell or a shock dose calculated in the wrong Atlantic dialect.
How accurate is the kidney approximation?
The 0.45 factor against the two lobe widths is the long-standing trade method and generally lands within roughly 10 percent of a measured fill on typical kidney outlines, which is adequate for dosing and chemical planning, the purposes volumes serve week to week. It is not adequate for ordering a made-to-measure liner or disputing a water bill to the gallon, and the page says so in the shape’s own name: standard approximation, used knowingly.
Related calculators
Method: shape geometry over the average of shallow and deep water depths (linear-slope method), the trade 0.45 approximation for kidney shapes, and exact gallon and litre conversions. Last reviewed: 15 October 2026.