How to use the oval pool volume calculator
Enter the inside length, the inside width and the water depth at the shallow and deep ends. For a flat-bottomed above-ground oval, put the same depth in both boxes. The calculator treats the outline as a true ellipse and returns gallons and liters.
Before you trust the result, look at the long sides of your pool. If they curve continuously from end to end, the ellipse answer is right. If they run straight for a stretch before curving into rounded ends, use the straight-sided method below.
What is the formula for an oval pool?
Gallons = length × width × average depth × 5.9, with all measurements in feet. The area of an ellipse is π ÷ 4 × length × width, about 0.785 × length × width, and multiplying by 7.48 gallons per cubic foot gives 5.875, rounded to 5.9.
For liters, multiply gallons by 3.785. The NIST conversion factors for US units define one US gallon as 3.785 412 liters, so the rounding never matters for pool work.
Worked example: 15 × 30 ft oval, 4 ft of water
- Ellipse area: 0.785 × 15 × 30 = about 353.4 sq ft.
- Cubic feet: 353.4 × 4 = about 1,413.7 cu ft.
- Gallons: 1,413.7 × 7.48 = about 10,575 gallons.
- Liters: 10,575 × 3.785 = about 40,032 liters.
At 4.5 ft of water the same pool holds about 11,897 gallons.
Is my pool a true oval or a straight-sided oval?
Many oval pools, including a lot of above-ground ovals, are not true ellipses. They are a rectangle in the middle with a half-circle on each end, sometimes called a racetrack or stadium shape. That shape fills more of its length × width box than an ellipse does, so the ellipse formula reads low.
The quick test: stand at one end and sight down a long side. If you see a straight wall section before the curve begins, treat it as straight-sided.
Straight-sided oval formula
Split the pool into a rectangle and one full circle (the two half-circle ends together):
- Rectangle: width × (length − width)
- Circle: 0.785 × width × width
- Gallons: (rectangle + circle) × average depth × 7.48
Worked example: straight-sided 15 × 30 ft oval, 4 ft of water
- Rectangle: 15 × (30 − 15) = 225 sq ft.
- Circle from the two ends: 0.785 × 15 × 15 = about 176.7 sq ft.
- Total area: about 401.7 sq ft.
- Gallons: 401.7 × 4 × 7.48 = about 12,020 gallons.
That is about 1,445 gallons, or 13.7%, more than the true-ellipse answer for the same length and width. A straight-sided 12 × 24 ft oval at 4 ft holds about 7,693 gallons versus 6,768 for a true ellipse, and an 18 × 33 ft at 4.5 ft holds about 17,655 versus 15,704.
If the straight section is shorter than the length minus the width, the true volume falls somewhere between the two answers. Measure the straight stretch and calculate the pieces directly: straight length × width for the middle, plus whatever the ends add.
How do I measure an oval pool?
Measure inside the wall at the waterline. Length is the longest distance end to end through the center; width is the widest distance side to side, measured at right angles to the length at the midpoint.
- Skip the uprights and top rails. On above-ground ovals the support structure along the sides sits outside the water.
- Measure the width at the middle, not near the ends, where the curve has already begun.
- Check that the long sides are parallel. If one side bows in, measure the width in two or three places and average.
- Record the depth at both ends of an inground oval. Above-ground ovals are usually flat, so one depth reading is enough.
Inground oval pools with sloped floors
Inground ovals often slope from a shallow end to a deep end like a rectangle does. Use (shallow + deep) ÷ 2 for the average depth, as long as the slope is even.
Worked example: 16 × 32 ft inground oval, 3.5 to 7 ft deep
- Average depth: (3.5 + 7) ÷ 2 = 5.25 ft.
- Ellipse area: 0.785 × 16 × 32 = about 402.1 sq ft.
- Gallons: 402.1 × 5.25 × 7.48 = about 15,792 gallons (about 59,781 liters).
If the deep end is a short hopper rather than an even slope, take several depth readings down the length and average them instead of using the two end depths.
Oval pool sizes: how much does each step up add?
Because both length and width grow together, each standard oval size adds a big jump in water. Using ellipse math:
- 12 × 24 ft at 4 ft of water: about 6,768 gallons
- 15 × 30 ft at 4 ft: about 10,575 gallons
- 16 × 32 ft at 4.5 ft: about 13,536 gallons
- 18 × 33 ft at 4.5 ft: about 15,704 gallons
- 18 × 40 ft at 4.5 ft: about 19,036 gallons
A 16 × 32 ft oval at 4.5 ft holds almost exactly the same water as a 24 ft round pool at 4 ft, a useful reminder that footprint alone tells you little. For top-offs, a 15 × 30 ft oval takes about 220 gallons per inch of depth.
Worked example in meters
A 4.5 × 9 m oval with 1.2 m of water: 0.785 × 4.5 × 9 × 1.2 = about 38.17 m³, or 38,170 liters (about 10,084 US gallons). In metric there is no 7.48 step: cubic meters × 1,000 gives liters directly.
Common mistakes with oval pools
- Using the rectangle formula. It overstates a true oval by about 27%: a 15 × 30 at 4 ft becomes 13,465 gallons instead of 10,575.
- Using the ellipse formula on a straight-sided oval. It understates by about 14% for a pool twice as long as it is wide, and by more for longer, narrower ovals.
- Measuring to the outside of the frame. The uprights and rails add width that holds no water.
- Using wall height instead of water depth. On above-ground ovals the water sits several inches below the top rail.
- Measuring width near an end. That catches the curve and reads narrow.
For other shapes, or to check a pool that combines an oval with a rectangular section, go back to the main pool volume calculator.