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How to Calculate Pool Volume
The volume of a rectangular pool is length times width times average depth. To convert the result from cubic feet to gallons, multiply by 7.48:
\[\text{Volume (gallons)} = L \times W \times D_{\text{avg}} \times 7.48\]A pool that is 30 feet long, 15 feet wide, and averages 5 feet deep holds 30 x 15 x 5 x 7.48 = 16,830 gallons. Knowing this number is necessary for chemical dosing, heater sizing, pump selection, and estimating how long it takes to fill or drain.
Why average depth matters
Most pools are not a uniform depth. A typical residential pool slopes from a 3-foot shallow end to an 8-foot deep end. Using the maximum depth would overestimate the volume, and using the minimum would underestimate it.
Average depth is the midpoint between the shallowest and deepest points:
\[D_{\text{avg}} = \frac{D_{\text{shallow}} + D_{\text{deep}}}{2}\]For a pool with a 3-foot shallow end and an 8-foot deep end, the average depth is (3 + 8) / 2 = 5.5 feet. This works well when the pool floor slopes at a constant grade from one end to the other. If the pool has a flat shallow section, a steep drop-off, and a flat deep section, the average still gives a reasonable estimate, but it will be slightly less accurate than calculating each section separately.
Rectangular pool worked example
A backyard pool measures 32 feet long, 16 feet wide, with a 3.5-foot shallow end and a 7.5-foot deep end.
Average depth: (3.5 + 7.5) / 2 = 5.5 feet.
\[V = 32 \times 16 \times 5.5 = 2{,}816 \text{ cubic feet}\] \[V = 2{,}816 \times 7.48 = \textbf{21,064 gallons}\]That figure determines how much chlorine, acid, or other chemicals the pool needs. Chemical dosing charts are always based on gallons, so getting this number right prevents over-treating or under-treating the water.
Round pool formula
For a circular pool, the area of the surface is pi times the radius squared. Multiply by average depth to get cubic feet, then by 7.48 for gallons:
\[\text{Volume (gallons)} = \pi \times r^2 \times D_{\text{avg}} \times 7.48\]If you know the diameter rather than the radius, divide by 2 first. A round pool with a 24-foot diameter and a constant depth of 4 feet:
\[r = 24 / 2 = 12 \text{ feet}\] \[V = \pi \times 12^2 \times 4 = \pi \times 144 \times 4 = 1{,}809.6 \text{ cubic feet}\] \[V = 1{,}809.6 \times 7.48 = \textbf{13,536 gallons}\]Above-ground pools are almost always round and uniform in depth, so this formula applies directly without needing to calculate average depth. The volume calculator handles cylinders and other shapes if you prefer to skip the manual arithmetic.
Oval pool formula
An oval (elliptical) pool uses a similar formula, substituting the two radii (half of the long axis and half of the short axis) for the single radius:
\[\text{Volume (gallons)} = \pi \times r_{\text{long}} \times r_{\text{short}} \times D_{\text{avg}} \times 7.48\]An oval pool 30 feet long, 15 feet wide, with an average depth of 5 feet:
\[V = \pi \times 15 \times 7.5 \times 5 = 1{,}767.1 \text{ cubic feet}\] \[V = 1{,}767.1 \times 7.48 = \textbf{13,218 gallons}\]Notice that the oval pool holds less water than a rectangular pool with the same outer dimensions. The curved walls reduce the area compared to a full rectangle.
Irregular and freeform pools
Kidney-shaped, L-shaped, and other freeform pools do not fit a single formula. The practical approach is to divide the pool into simpler shapes, calculate each section separately, and add the results.
For an L-shaped pool, measure the two rectangular sections independently. If one section is 20 x 10 feet at 4 feet deep and the other is 12 x 10 feet at 6 feet deep:
Section 1: 20 x 10 x 4 x 7.48 = 5,984 gallons
Section 2: 12 x 10 x 6 x 7.48 = 5,386 gallons
Total: 5,984 + 5,386 = 11,370 gallons
For kidney-shaped pools, a common approximation is to treat the pool as an oval with a correction factor. Measure the longest dimension (A), the widest dimension (B), and average depth, then use:
\[\text{Volume (gallons)} = A \times B \times D_{\text{avg}} \times 0.45 \times 7.48\]The 0.45 factor accounts for the curved indentation that makes a kidney shape smaller than a full oval. This is an approximation, accurate to within about 10% for most kidney-shaped residential pools.
Converting between units
The 7.48 multiplier converts cubic feet to US gallons. Here are the other common conversions:
| From | To | Multiply by |
|---|---|---|
| Cubic feet | US gallons | 7.48 |
| Cubic feet | Liters | 28.317 |
| Cubic meters | US gallons | 264.17 |
| Cubic meters | Liters | 1,000 |
If you measured your pool in meters instead of feet, calculate the volume in cubic meters first (length x width x average depth, all in meters), then multiply by 264.17 for gallons or by 1,000 for liters.
Why you need to know your pool volume
Chemical dosing. Chlorine, pH adjusters, algaecides, and shock treatments are all dosed by gallons. A pool owner adding chlorine to a 15,000-gallon pool needs a different amount than one treating a 25,000-gallon pool. Underdosing leaves the water unsanitized; overdosing wastes chemicals and can irritate skin and eyes.
Heater sizing. Pool heaters are rated in BTUs, and the time to raise water temperature depends on volume. A heater that warms a 15,000-gallon pool by one degree per hour would take nearly twice as long on a 28,000-gallon pool. Knowing the volume before buying a heater avoids purchasing one that is too small for the job.
Pump and filter sizing. Pool pumps should turn over the entire volume at least once every 8 to 12 hours. A 20,000-gallon pool needs a pump that moves at least 20,000 / 8 = 2,500 gallons per hour (about 42 gallons per minute). An undersized pump cannot circulate the water properly, leading to poor filtration and algae growth.
Fill time. A standard garden hose delivers about 9 gallons per minute. Filling a 20,000-gallon pool from empty would take 20,000 / 9 = approximately 2,222 minutes, or about 37 hours. Knowing this in advance helps with planning and water-bill estimates.
Quick reference for common pool sizes
| Pool dimensions | Average depth | Approximate volume |
|---|---|---|
| 12 x 24 ft rectangular | 5 ft | 10,771 gallons |
| 16 x 32 ft rectangular | 5.5 ft | 21,064 gallons |
| 20 x 40 ft rectangular | 5.5 ft | 32,912 gallons |
| 18 ft round | 4 ft | 7,634 gallons |
| 24 ft round | 4 ft | 13,536 gallons |
| 15 x 30 ft oval | 5 ft | 13,218 gallons |
These figures assume a constant slope from shallow to deep end. Pools with flat bottoms, benches, or diving wells will differ. For an exact calculation with your measurements, the volume calculator accepts custom dimensions for several common shapes.