How Much Water Is in My Pool? (Gallons)

By RugCalc Editorial Team · Last updated: 2026-10-13 · 8 min read

Quick answer: pool volume (cu ft) × 7.48 = US gallons. A 20×40 ft rectangular pool at 5 ft deep holds about 30,000 gallons. Here's the full method by shape.

Gallons per cubic foot

The key number: 1 cubic foot of water = 7.48 US gallons. So find the pool's volume in cubic feet, then multiply by 7.48. (Note: 7.48 is US gallons — imperial gallons are different, about 6.23 per cubic foot. If you're outside the US, confirm which gallon your local treatments are dosed in before you use this number.)

Knowing your pool's volume in gallons matters for two things more than anything else: chemical dosing (sanitizers, algaecides, and shock are all calculated per gallon) and fill time. Guessing high by 20% on a 30,000-gallon pool means under-dosing chemicals, which is exactly when algae gets a foothold.

Rectangular pools

Length × Width × Average Depth × 7.48 = gallons. If the bottom slopes, use the average of the shallow and deep ends rather than a single depth. For a pool that goes 3 ft at one end and 6 ft at the other, average depth is (3 + 6) ÷ 2 = 4.5 ft.

Measure to the actual water line, not the top of the wall — a pool with a 6-ft wall only filled to 5 ft holds 5 ft of water. This is a very common reason people overestimate volume.

Round pools

Diameter × Diameter × Depth × 5.9 = gallons. The 5.9 factor already folds in π/4 × 7.48, so for round pools multiply by 5.9 instead of 7.48. Use the diameter at the water line and a single average depth for the whole pool.

Oval pools

Approximate an oval as a rectangle for a quick estimate, or use the formula: Length × Width × Depth × 5.9. The width here is the width of the oval's widest point. The ×0.8 oval factor you'll sometimes see is a shortcut that handles gently-curved oval pools closely; for accuracy, stick to the length × width × depth × 5.9 approach.

Worked example

• Rectangular pool = 20 × 40 × 5 ft = 4,000 cu ft
• Gallons = 4,000 × 7.48 = 29,920 US gallons

At a typical fill rate of ~10 gallons per minute, that's about 50 hours to fill — often done over a couple of days to avoid stressing the water supply. Run it overnight and check for leaks at the same time.

Common mistakes

• Using one depth on a sloped pool. Average the shallow and deep ends.
• Mixing US and imperial gallons. 7.48 vs 6.23 per cubic foot — know which you're using.
• Using the round-pool factor on a rectangle. The 5.9 factor only applies to circular shapes.
• Ignoring chemical dosing. Pool treatments are dosed per gallon — an accurate volume matters.
• Measuring to the top of the wall. Use the actual water line, not the full wall height.

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