How Many Gallons of Water Are in My Pool? (2026 Guide)

The rectangular pool formula

Find the surface area in square feet, multiply by the average depth in feet, then convert cubic feet to gallons — one cubic foot holds 7.48 US gallons:

Gallons = length (ft) × width (ft) × average depth (ft) × 7.48

Most pools slope from a shallow end to a deep end, so use average depth = (shallow depth + deep depth) ÷ 2; a pool that goes from 3 ft to 6 ft averages 4.5 ft. Treat the result as an estimate — hopper bottoms, coves and your actual waterline shift the true volume by a few percent.

Worked example: a 20×40 ft pool

Take an average in-ground pool, 20 ft by 40 ft, sloping from a 3 ft shallow end to a 6 ft deep end:

• Average depth: (3 + 6) ÷ 2 = 4.5 ft
• Surface area: 20 × 40 = 800 sq ft
• Gallons: 800 × 4.5 × 7.48 ≈ 26,928

So the pool holds about 26,900 gallons of water. Run your own numbers through the pool volume calculator and it does the averaging and unit conversion for you.

Round and oval pools

For a round pool, use the radius — half the diameter — squared: Gallons = π × (diameter ÷ 2)² × average depth × 7.48, with π ≈ 3.14. An 18-ft round pool that averages 4.5 ft deep holds about 8,600 gallons.

An oval pool behaves like a rectangle with the corners rounded off, so it holds roughly 10% less than a same-size rectangle: Gallons ≈ length × width × average depth × 7.48 × 0.9. A 16×32 ft oval at 4.5 ft deep comes to about 15,500 gallons.

Gallons for common pool sizes

All figures assume a 4.5 ft average depth and are rounded to the nearest 100 gallons:

Unrounded, those five figures are 17,234, 21,812, 26,928, 8,565 and 15,511 gallons. If your pool is a foot deeper or shallower on average, add or subtract roughly 7.5% per foot of depth change.

Pool-sizing tips that save you from guesswork

• Dose chemicals by gallons, not surface area — shock, algaecide and salt labels quote amounts per 10,000 gallons, so a wrong volume estimate under- or over-treats the water.
• Measure both ends before averaging — write down the shallow end and the deep end water depths at the skimmer and the main drain, then apply (shallow + deep) ÷ 2. Guessing the deep end is the biggest single source of error.
• Split irregular pools into shapes and add — break a freeform or L-shaped pool into rectangles, circles and wedges, give each section its own average depth, then sum the gallons.
• Estimate fill cost separately — a 26,900-gallon refill is billed at your local rate per 1,000 gallons, so the same math doubles as a fill-cost estimate before you order a water delivery.

Pool math rarely stops at the waterline: a pump that runs eight hours a day is one of the bigger loads on a home, so if you are weighing offsets, see how many solar panels you need to run it. And if the project moves outdoors onto the deck or driveway, the same estimate-by-area approach carries over to gravel for a driveway.

Calculate your pool's volume →