What this calculator works out
Chemicals, pump sizing and filter selection are all dosed or rated against one number: how much water the pool actually holds. Guessing that figure, or reading it off a builder's brochure for a "similar" size, is one of the most common reasons a pool runs cloudy or over-chlorinated. This calculator takes your pool's shape, its measurements and its depth (constant, or sloped from a shallow end to a deep end) and works out the volume in US gallons, the figure every US pool chemical label and pump spec is written against, alongside UK litres and imperial gallons for anyone working from a UK product label instead. It also turns that volume into a rough chemical dosing guide and the pump flow rate an honest turnover needs.
How the maths works, shape by shape
Every shape reduces to the same two steps: work out the surface area, then multiply by the average depth to get a volume in cubic feet. A rectangular pool is the simplest case, length times width.
A round pool is a circle, so its area follows directly from the radius, half the diameter.
An oval pool is treated as an ellipse: measure the long axis and the short axis at their widest points, at right angles to each other, and the area is a quarter of pi times the two multiplied together. This is exact geometry, not an approximation, the same as the rectangle and round formulas.
Once the area is known, volume follows the same last step for every shape: multiply by the average depth, then convert cubic feet into US gallons using the exact 7.48 gallons-per-cubic-foot constant (1,728 cubic inches in a cubic foot, divided by the 231 cubic inches in a US gallon).
Working the default rectangle through: a 32 × 16ft pool with a 3ft shallow end and an 8ft deep end. Area is 32 × 16, or 512 square feet. Average depth is (3 + 8) ÷ 2, or 5.5ft. Volume is 512 × 5.5, or 2,816 cubic feet, which comes to about 21,065 US gallons, roughly 79,740 litres or 17,540 UK gallons.
Shallow end, deep end and average depth
Very few pools are one constant depth all over. Where the floor slopes from a shallow end to a deep end, this calculator uses the standard published convention: add the shallow depth to the deep depth and divide by two. A pool with a 3ft shallow end and an 8ft deep end averages to 5.5ft, not 3ft and not 8ft. This assumes a reasonably even slope between the two measured points; a pool with a flat shallow shelf, then a sudden drop to a deeper main body, holds slightly more water than this simple average suggests. For that kind of layout, the closest honest estimate is to measure the shelf and the deep section as two separate constant-depth zones and add their volumes together.
Kidney and freeform shapes: an honest approximation
A kidney or freeform pool has no clean geometric formula the way a rectangle, circle or ellipse does, so every pool-industry calculator falls back on an estimate: measure the pool's overall length and width at the widest points, as though drawing a rectangle around it, then apply a shape multiplier for the curved sides taking up less water than a full rectangle would. This is genuinely the least precise of the four shapes here, and published sources do not agree on one figure: some quote roughly 0.85, treating the pool as a general freeform shape holding about 85% of the bounding rectangle, while others publish a lower figure nearer 0.45 to 0.585 when two separate end widths are measured and combined. This calculator uses the single-width 0.85 convention, matching the one-length, one-width inputs above, and states that spread honestly rather than presenting a false level of precision. Treat the kidney result as a planning estimate and confirm the exact figure against your pool builder's own specification if you need it to the gallon.
US gallons, UK litres and the weight of all that water
US and UK gallons are not the same size, which trips people up constantly when a chemical label or a filter spec is written for the other market. A US gallon is defined as 231 cubic inches; a UK (imperial) gallon has been fixed at exactly 4.54609 litres since the Weights and Measures Act 1985 and works out about a fifth larger. This calculator shows both, plus the plain litre figure, so whichever product label or pump spec you're reading from converts straight across. Water is also heavy: at roughly 8.34 lb per US gallon (1 litre weighs almost exactly 1kg), a modest 21,000-gallon backyard pool holds close to 175,000lb of water, over 87 tons, which is part of why an above-ground pool's siting and base preparation matter as much as the liner itself.
Chlorine, shock dosing and why gallons matter
Every pool chemical dose on a product label is written per gallon, so the volume figure above is the number that turns a bag of shock into an actual amount to add. Routine free chlorine is generally kept in a 1 to 3 ppm range for day-to-day sanitising. Shocking the pool, a heavier one-off dose used weekly or after heavy bather load, cloudy water or algae, follows a widely published convention of roughly 1 lb of calcium hypochlorite (cal-hypo) per 10,000 gallons, which raises free chlorine by around 8 ppm in a typical pool; an algae bloom or a badly neglected pool often needs two to three times that routine dose. Cal-hypo strength varies by product, most run 65 to 73% available chlorine, so these figures are a sound starting point rather than an exact substitute for your own product's printed dosing instructions, which this calculator cannot see.
Salt pools still need this number
A saltwater pool does not skip chlorine, it generates it on site: a chlorinator cell runs a low-voltage current through the salted water, producing free chlorine by electrolysis, which then sanitises the pool exactly the same way chlorine added by hand does. The 1 to 3 ppm free chlorine target above still applies, and the cell's output setting is calibrated against the pool's volume in gallons much the same way a hand-dosed routine is. Salt pools still need an occasional manual shock too, particularly after an algae bloom or heavy rain, since a chlorinator cell is rarely sized to produce a sudden heavy dose on demand.
Sizing a pump: the turnover rule
A pump and filter are sized against how long it takes to pass the entire pool volume through them once, called the turnover time. Published guidance for a residential pool generally sits in a 6 to 10 hour window, commonly centred on 8 hours as a practical middle ground between water clarity and running cost; public and commercial pools are usually held to a stricter 4 to 6 hour standard under most health codes. This calculator shows the flow rate needed at 6, 8 and 10 hours side by side, so a faster or slower target is easy to compare.
A pump's rated flow on its nameplate also tends to overstate what actually reaches the pool, since real plumbing and a dirty filter add resistance a free-flow lab rating doesn't account for; sizing a little above the bare calculated figure is standard trade practice for that reason.
Questions people ask
Why do I need to know my pool's gallons before adding chemicals?
Every chemical dose, from routine chlorine to a shock treatment, is written per gallon on the product label. Without the volume figure, there is no way to know whether a dose is too weak to do anything or strong enough to over-chlorinate the water, both of which are common and avoidable mistakes.
How accurate is the kidney shape formula?
Less accurate than the other three shapes, honestly. A kidney or freeform pool has no exact geometric formula, so this calculator applies the widely used 0.85 x length x width estimate, though published sources vary between roughly 0.45 and 0.85 depending on how the width is measured. Treat a kidney result as a solid planning figure for chemicals and pump sizing, not an exact number.
Do saltwater pools still need this chemical dosing information?
Yes. A salt system generates chlorine on site rather than skipping it, so the same 1 to 3 ppm free chlorine target applies, and the chlorinator's output is set against the pool's gallons in much the same way a hand-dosed pool's chemical routine is. Salt pools still need an occasional manual shock too, since the cell is rarely sized to deliver a sudden heavy dose on its own.
How often should my pool water turn over?
Residential pools are generally guided toward a 6 to 10 hour turnover, with 8 hours a common practical target; commercial and public pools are usually held to a stricter 4 to 6 hours under most health codes. Divide the pool's volume by the target turnover time in minutes to get the flow rate a pump needs to deliver.
What's the difference between constant depth and average depth?
A constant-depth pool is the same depth everywhere, so that single measurement is used directly. A sloped pool, with a shallower end and a deeper end, uses the average of the two, (shallow + deep) ÷ 2, the standard convention published across the pool industry for a reasonably even slope.
How much does a full pool of water actually weigh?
Roughly 8.34lb per US gallon, so a modest 21,000-gallon backyard pool holds close to 175,000lb of water, over 87 tons. That weight is one reason siting, base preparation and structural support matter as much as the liner or shell for an above-ground or partly buried pool.
Shape formulas on this page follow the conventions published by pool-industry suppliers and calculators, including Doheny's Pool Supplies and Inch Calculator, cross-checked against Swim University and Pool Nerd; the kidney/freeform multiplier varies between publishers (roughly 0.45 to 0.85) and is stated as an approximation throughout. Pump turnover guidance follows the widely cited 6 to 10 hour residential range (Leslie's, Lotus Pools and general health-code summaries). Chemical dosing follows the commonly published 1 lb cal-hypo per 10,000 gallons convention (Swim University, AquaDoc); actual dosing always depends on your product's own printed strength and instructions, which this calculator does not see.