The quick answer: multiply the pool's surface area by its average water depth, then convert to gallons:
- Rectangle: length × width × average depth × 7.48
- Round: diameter × diameter × depth × 5.9
- Oval: length × width × depth × 5.9
All measurements are in feet. For a 16 × 32 ft pool averaging 5 ft deep, that's about 19,150 gallons. Pick your shape in the calculator below; it also works in metres and litres.
Why volume matters more than people think
Every dose calculation, whether chlorine, acid, salt or stabilizer, scales with volume. If you guess 15,000 gallons and the pool actually holds 20,000, every dose is 25% short. You'll think the chemicals "don't work", buy more, and chase numbers all season. Measuring once fixes that for good.
Formulas by shape (feet → US gallons)
| Shape | Formula | Example |
|---|---|---|
| Rectangle / square | L × W × avg depth × 7.48 | 16 × 32 × 5 → 19,150 gal |
| Round | D × D × depth × 5.9 | 18 ft at 4 ft → 7,610 gal |
| Oval | L × W × depth × 5.9 | 15 × 30 at 4 ft → 10,580 gal |
| Kidney | (A + B) × L × avg depth × 3.37 | A=10, B=16, L=32, 5 ft → 14,000 gal |
The gallons shown are computed with the exact factors (π/4 × 7.48 = 5.875 for round and oval), which is why they come out a little below the 5.9 rule of thumb. | L-shape | Split into two rectangles and add them | | | Freeform / irregular | Split into rectangles, circles and ovals, then add; or verify with chemistry (below) | |
Where the numbers come from:
- 1 cubic foot holds 7.48 US gallons.
- A circle's area is π/4 × D², so a round pool is 0.785 × 7.48 ≈ 5.9.
- The oval formula treats the pool as an ellipse (area π/4 × L × W), so it uses the same 5.9 factor. That's what pool tables use for "oval" above-ground pools. A pool with straight sides and half-circle ends (a stadium shape) holds more: measure it as a rectangle plus one round pool with the width as diameter. For example, a 15 × 30 ft stadium-shaped pool at 4 ft holds about 12,020 gallons, against 10,580 for the ellipse.
- For a kidney, A and B are the widths across the two lobes and L is the length.
Metric (metres → litres)
- Rectangle: L × W × avg depth = m³; × 1,000 = litres. An 8 × 4 m pool at 1.5 m average holds 48,000 L.
- Round: 0.785 × D × D × depth. A 5.5 m pool at 1.2 m holds about 28,500 L.
- Converting: 1 US gallon = 3.785 L; 1 m³ = 1,000 L ≈ 264 US gallons.
Measuring average depth
- Even slope: (shallow depth + deep depth) ÷ 2. Measure water depth, from the waterline down, not to the top of the wall.
- Shallow end, slope, then a hopper deep end: split the pool into sections along its length. Calculate each one with its own average depth and add them up. Average depth can be overestimated by a lot if a short, deep hopper is treated as half the pool.
- Above-ground pools: the wall height (48", 52", 54") is not the water depth. Most are filled to several inches below the top. Measure the water, or use your manufacturer's stated capacity at the fill line.
Common above-ground sizes
These assume a uniform depth. Measure yours.
| Pool | At 3.5 ft water | At 4 ft water |
|---|---|---|
| 12 ft round | 2,960 gal | 3,380 gal |
| 15 ft round | 4,630 gal | 5,290 gal |
| 18 ft round | 6,660 gal | 7,610 gal |
| 21 ft round | 9,070 gal | 10,360 gal |
| 24 ft round | 11,840 gal | 13,540 gal |
| 27 ft round | 14,990 gal | 17,130 gal |
| 15 × 30 ft oval | 9,250 gal | 10,580 gal |
| 18 × 33 ft oval | 12,210 gal | 13,960 gal |
Pool sizes are nominal. A "15 ft" pool may be slightly smaller inside the wall, and frame pools bulge a little when full. For the most accurate number, verify it.
Verify your volume with chemistry (the pro trick)
Measurements on curved or freeform pools can be off by 20% or more. You can check your volume by adding a known amount of something and measuring how much the level rises.
With salt (salt water pools, or any pool you don't mind adding salt to):
- Test salt accurately. A good meter or a titration test is best.
- Add a known weight, say 80 lb. Let it fully dissolve and circulate for 24 hours.
- Test again. Volume in gallons ≈ pounds added × 120,000 ÷ ppm rise. Example: 80 lb raising salt by 500 ppm → about 19,200 gallons.
With stabilizer (CYA), if you were going to add some anyway:
- Add a known amount of stabilizer.
- Wait a week for it to dissolve, then measure the rise. 13.5 oz of 99% stabilizer granules raises CYA 10 ppm in 10,000 gallons.
Test error matters here. Use a big enough addition that the rise is well beyond your test's precision, and treat the result as a sanity check on your measurements.
Gallons ↔ litres quick reference
| US gallons | Litres |
|---|---|
| 5,000 | 18,900 |
| 10,000 | 37,850 |
| 15,000 | 56,800 |
| 20,000 | 75,700 |
| 25,000 | 94,600 |
| 30,000 | 113,600 |
Common mistakes
- Using wall height instead of water depth on above-ground pools.
- Treating a steep deep end as if the whole pool sloped evenly.
- Using the builder's estimate from years ago. Renovations, steps and benches change it.
- Mixing units: inches, feet and metres in one formula.
- Forgetting the spa if it spills into the pool and shares the water. Add its volume.
Set it once in Pool Calc
Set your shape and measurements once in Pool Calc and every dose from then on uses them. You can switch between gallons and litres at any time, and the app always shows the unit next to the number. If you verify your volume with a salt or CYA test, you can enter the corrected number and your doses update everywhere.
One pool? Every feature, free. No account, no ads, and iCloud backup is included.
Safety
- Keep chemicals out of reach of children and pets, and keep them out of the water until it is back in range.
- Never mix chemicals. Never add acid and chlorine together: mixing them releases toxic chlorine gas.
- Add chemicals to water, never water to chemicals.
- Store chemicals sealed, cool and dry, apart from each other.
- Read and follow the product label. It always comes first.