Pool Water Volume Calculator in Gallons

This calculator works out the volume of water in a swimming pool in US gallons. You enter the pool’s shape, dimensions, and average depth, and it returns the total gallons, along with the volume in cubic feet and liters. The result is the amount of water needed to fill the pool to the specified depth.

Formula

The calculator uses the standard geometric volume formulas for each shape, then converts cubic feet to gallons.

Rectangular pool: Volume (ft³) = Length × Width × Average Depth

Circular pool: Volume (ft³) = π × (Diameter/2)² × Average Depth

Oval pool: Volume (ft³) = π × (Length/2) × (Width/2) × Average Depth

Average Depth = (Shallow End Depth + Deep End Depth) / 2

Gallons = Volume (ft³) × 7.48052

Where:

• Length, Width, Diameter are in feet (ft)
• Depths are in feet (ft)
• 7.48052 is the number of US gallons in one cubic foot
• Ï€ ≈ 3.14159

Worked Example

Using the default values: a rectangular pool, 30 ft long, 15 ft wide, shallow end 3 ft, deep end 6 ft.

1. Average depth = (3 + 6) / 2 = 4.5 ft
2. Volume in cubic feet = 30 × 15 × 4.5 = 2,025 ft³
3. Gallons = 2,025 × 7.48052 = 15,148.05 US gallons

The calculator rounds to 15,148 gallons. The same volume in liters is 2,025 × 28.3168 = 57,341 L (rounded).

Why Pool Volume Matters

Knowing your pool’s water volume in gallons is essential for several practical reasons:

• Chemical dosing: Most pool chemicals (chlorine, pH adjusters, algaecides) are measured per 10,000 gallons. Adding too little fails to sanitize, too much can irritate skin or damage equipment.
• Filtration sizing: A pump and filter should circulate the entire pool volume in 8 hours. A 15,000-gallon pool needs a pump rated at least 31 gallons per minute.
• Heating costs: The energy required to raise water temperature depends directly on volume. A larger pool costs more to heat.
• Filling time: If your hose delivers 10 gallons per minute, a 15,000-gallon pool takes 25 hours to fill. Volume lets you plan.

How to Measure Your Pool

For accurate results, measure in feet and inches, then convert inches to decimal feet (e.g., 3 ft 6 in = 3.5 ft).

• Rectangular pools: Measure the longest length and widest width at the waterline, not the outer edge.
• Circular pools: Measure the diameter across the center. If you only have the circumference, divide by Ï€ (3.14159) to get diameter.
• Oval pools: Measure the longest length and the widest width at the center.
• Depth: Measure from the water surface to the pool floor. For sloped floors, take the deepest and shallowest points. For flat floors, use the same depth for both.

If your pool has an irregular shape (L-shaped, kidney, freeform), break it into rectangles and circles, calculate each section, then add the volumes. This calculator handles the three most common shapes; for irregular pools, use the rectangle formula with average width.

Common Mistakes and Misconceptions

• Using the wrong gallon: This calculator gives US gallons. Imperial gallons (used in the UK and Canada) are about 20% larger. 1 cubic foot = 6.228 Imperial gallons, not 7.48. If you need Imperial gallons, multiply the cubic feet by 6.228.
• Forgetting the average depth: A pool with a 3-ft shallow end and 9-ft deep end does not hold the same as a 6-ft flat pool? Actually it does, because the average is (3+9)/2 = 6. But many people mistakenly use the deep end only, overestimating by up to 50%.
• Measuring the outside dimensions: Pool walls are thick. Use the waterline dimensions, not the outer shell.
• Rounding too early: If you round the volume to the nearest thousand before chemical dosing, you might under- or over-dose. Keep at least two significant figures.

Limitations of the Formula

The formulas assume a perfectly geometric shape and a linear slope between shallow and deep ends. Real pools have curved floors, steps, and benches that reduce actual water volume. The result is an estimate, usually within 5–10% of the true volume. For exact chemical dosing, use a test kit and adjust based on your specific pool. The calculator also does not account for water displacement by objects (ladders, toys, people) or water loss due to evaporation and splashing.

Frequently Asked Questions

How many gallons is a 20×40 pool?

Assuming a rectangular pool with an average depth of 5 ft (e.g., 4 ft shallow, 6 ft deep), the volume is 20 × 40 × 5 = 4,000 ft³, which is 4,000 × 7.48052 = 29,922 US gallons. If the average depth is 6 ft, it’s 35,906 gallons.

How do I calculate gallons in a round pool?

Use the formula: π × (diameter/2)² × average depth × 7.48052. For a 24-ft round pool with 4 ft depth: π × 12² × 4 × 7.48052 = 3.14159 × 144 × 4 × 7.48052 = 13,593 gallons.

What is the difference between US and Imperial gallons?

US gallon is 231 cubic inches (3.785 liters). Imperial gallon is 277.42 cubic inches (4.546 liters). So 1 US gallon ≈ 0.833 Imperial gallons. A pool that holds 20,000 US gallons holds about 16,654 Imperial gallons.

How many gallons does a typical inground pool hold?

Residential inground pools range from about 15,000 to 30,000 gallons. A 16×32 ft pool with an average depth of 6 ft holds about 23,000 gallons. A smaller 12×24 ft pool with 5 ft average depth holds about 10,800 gallons.

Does the calculator account for an uneven bottom?

It uses the average of the shallow and deep ends, which assumes a straight slope. If your pool has a more complex bottom (multiple slopes, hopper shape), the estimate may be less accurate. In that case, divide the pool into sections and calculate each separately.

Can I use this for a spa or hot tub?

Yes, if the spa is rectangular or circular. For a circular spa, measure the diameter and depth. For example, a 7-ft round spa with 3 ft depth holds π × 3.5² × 3 × 7.48052 ≈ 864 gallons.

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sel shape | Pool shape | Rectangular=1, Circular=2, Oval=3
in length | Length (longest side) | ft | 30 | 0
in width | Width (shortest side) | ft | 15 | 0
in depth_shallow | Shallow end depth | ft | 3 | 0
in depth_deep | Deep end depth | ft | 6 | 0
out gallons | Water volume | US gal | (shape==1 ? length*width*(depth_shallow+depth_deep)/2 : shape==2 ? pi*(length/2)*(width/2)*(depth_shallow+depth_deep)/2 : pi*(length/2)*(width/2)*(depth_shallow+depth_deep)/2) * 7.48052 | 0
out cubic_ft | Volume | cu ft | (shape==1 ? length*width*(depth_shallow+depth_deep)/2 : shape==2 ? pi*(length/2)*(width/2)*(depth_shallow+depth_deep)/2 : pi*(length/2)*(width/2)*(depth_shallow+depth_deep)/2) | 0
out liters | Volume | L | (shape==1 ? length*width*(depth_shallow+depth_deep)/2 : shape==2 ? pi*(length/2)*(width/2)*(depth_shallow+depth_deep)/2 : pi*(length/2)*(width/2)*(depth_shallow+depth_deep)/2) * 28.3168 | 0