Your pool
| Measure | Value |
|---|---|
| Volume (cubic feet) | — |
| Volume (liters) | — |
| Water weight | — lbs |
| Fill time | — |
| Fill cost | — |
| Pump turnover target (8 hrs) | — GPH |
Chemical starter doses
| Chemical | Starting dose |
|---|---|
| Cal-hypo shock | — lbs (1 lb per 10,000 gal) |
| Baking soda | — lbs (raises alkalinity ≈10 ppm) |
| Soda ash | — oz (raises pH ≈0.2) |
| Calcium chloride | — lbs (raises hardness ≈10 ppm) |
| Cyanuric acid (stabilizer) | — oz (raises CYA ≈10 ppm; takes days to register) |
How the math works
Every pool chemical is dosed in parts per million, which means every dose is tied to your total water volume. Get the volume wrong and every treatment is wrong with it. The calculation itself is simple geometry plus one conversion factor: find the pool's volume in cubic feet, then multiply by 7.48052 — the exact number of US gallons in a cubic foot.
Rectangular pools
Length × width × average depth × 7.48052. Example: a 20 × 40 ft pool with a 3 ft shallow end and an 8 ft deep end. Average depth is (3 + 8) ÷ 2 = 5.5 ft, so 20 × 40 × 5.5 × 7.48052 = 32,914 gallons.
Round pools
π × radius² × average depth × 7.48052. Example: a 24 ft diameter pool at a constant 4 ft depth. Radius is 12 ft, so 3.1416 × 12² × 4 × 7.48052 = 13,536 gallons.
Oval pools
π × (length ÷ 2) × (width ÷ 2) × average depth × 7.48052. Example: a 30 × 15 ft oval with a 4.5 ft average depth gives 3.1416 × 15 × 7.5 × 4.5 × 7.48052 = 11,897 gallons. Note this treats the oval as a true ellipse — see the FAQ if your oval has straight sides.
Kidney and freeform pools
0.45 × (width A + width B) × length × average depth × 7.48052. Measure the widest point of each rounded lobe, perpendicular to the length. The 0.45 factor is the industry-standard adjustment for the concave curve, which shrinks the surface to roughly 45% of the bounding rectangle. Example: 32 ft long, 16 ft at the wide end, 12 ft at the narrow end, 5 ft average depth: 0.45 × 28 × 32 × 5 × 7.48052 = 15,081 gallons.
Why average depth is the whole game
The single biggest source of error in pool math isn't the shape — it's the depth. Some calculators use only your deep-end number. Take the rectangular example above: using the 8 ft deep end instead of the 5.5 ft average claims 47,875 gallons instead of 32,914. That's 45% too high, and it would waste roughly 1.5 extra pounds of shock on every single treatment.
Always average the shallow and deep ends: (shallow + deep) ÷ 2. If your floor drops in steps rather than a smooth slope, take several depth readings and average those. And measure the water depth from the waterline down — not the wall height, which includes inches of air that hold no water.
Common pool sizes
Sanity-check your result against these standard sizes (volumes in US gallons at the given average depth):
| Rectangular | 4 ft avg | 5 ft avg | 6 ft avg |
|---|---|---|---|
| 12 × 24 ft | 8,618 | 10,772 | 12,926 |
| 15 × 30 ft | 13,465 | 16,831 | 20,197 |
| 16 × 32 ft | 15,320 | 19,150 | 22,980 |
| 20 × 40 ft | 23,938 | 29,922 | 35,906 |
| Round (diameter) | 4 ft avg | 5 ft avg | 6 ft avg |
|---|---|---|---|
| 18 ft | 7,614 | 9,518 | 11,421 |
| 24 ft | 13,536 | 16,921 | 20,305 |
| 27 ft | 17,132 | 21,415 | 25,698 |
For context, an Olympic competition pool holds about 660,000 gallons — a typical 16,000-gallon family pool is roughly 2.4% of that, or about 200 full bathtubs.
Measuring tips
- Rectangular: measure the inside wall, at the waterline. If the pool has rounded corners, knock a few percent off.
- Round: measure straight across the middle, wall to wall. The radius is half of whatever you measure.
- Oval: length end to end, width across the narrow middle — both at the waterline.
- Kidney: measure both widths perpendicular to the length. Measuring at a diagonal inflates the width by 5–12%.
- Depth: use a pool pole or tape from the waterline to the floor, at the shallowest and deepest spots.
Metric note: one US gallon is 3.78541 liters, and the results table shows both. Everything on this page runs in your browser — no measurements ever leave your device.
Frequently asked questions
How do I figure out my pool's average depth?
Measure the water depth at the shallow end and at the deep end, then average the two: (shallow + deep) ÷ 2. If the floor slopes evenly, that average is a good estimate. If the floor drops in steps or has a deep well, take several depth readings, average them all, and use that number instead.
Can this calculator handle kidney-shaped or freeform pools?
Yes. Select the kidney shape and enter two widths: the widest point of each rounded end, measured perpendicular to the pool's length. The calculator applies the industry-standard 0.45 shape factor, which accounts for the concave curve reducing the surface area to roughly 45% of the bounding rectangle.
My above-ground oval has straight sides with round ends. Is the math right for me?
Mostly, but be aware this calculator treats an oval as a true ellipse. Many above-ground ovals are actually capsule-shaped (straight sides with semicircular ends), which holds about 10–15% more water than the ellipse math says. For a capsule oval, the ellipse result is a conservative estimate — fine for buying chemicals, but size your pump for the slightly larger number.
How accurate is the gallon estimate?
Within a few percent for pools with evenly sloping floors, assuming your measurements are right. Steps, benches, swim-up shelves, and rounded corners displace water, so real volume is usually a touch lower than the geometric estimate. For chemicals, the calculator's number is the safe, slightly conservative starting point. Always confirm with a water test before dosing.
How much shock does my pool need?
A widely published starting point is 1 lb of calcium hypochlorite shock per 10,000 gallons of water. The calculator scales that to your pool and rounds to the nearest quarter-pound. Algae blooms, storms, or heavy swimmer load can call for more, so test your free chlorine and follow the product label.
How long does it take to fill a pool with a garden hose?
A typical garden hose flows about 9 gallons per minute, or 540 gallons per hour, so a 20,000-gallon pool takes roughly 37 hours of steady filling. Enter your hose's actual flow rate — time a 5-gallon bucket fill and divide 300 by the seconds it took — for a number matched to your setup.
Why do different pool calculators give different answers?
Three common reasons. First, some calculators use only the deep-end depth instead of the average, which inflates volume by 25–50%. Second, some round the conversion factor to 7.5 instead of the exact 7.48052 gallons per cubic foot. Third, oval and kidney formulas differ between sites. This calculator uses average depth, the exact conversion factor, and the standard ellipse and 0.45-factor formulas.
Should I measure the pool wall height or the actual water depth?
Measure the actual water depth, from the waterline down to the floor. The wall height includes the few inches of air above the waterline, which would overstate your volume. For width and length, measure across the waterline too, and for kidney pools keep your tape perpendicular to the length — measuring at a diagonal inflates the width.