Pool Gallons Calculator

Pool gallons calculator formula

A pool gallons calculator turns pool dimensions into the volume of water. The formula is gallons = length × width × average depth (all in feet) × 7.48.

There are 7.48 US gallons in one cubic foot. The first part of the formula gives cubic feet, and the 7.48 turns it into gallons. This calculator is built for rectangular pools. For a round or oval pool, see the notes below.

Worked examples

Example 1 is the default pool, 20 ft × 10 ft with a 60 inch average depth.

• 60 inches ÷ 12 = 5 ft
• Cubic feet: 20 × 10 × 5 = 1,000
• Gallons: 1,000 × 7.48 = 7,480
• Water at $0.01 per gallon: 7,480 × $0.01 = $74.80

Example 2 is a larger pool, 32 ft × 16 ft, with a 3.5 ft shallow end and an 8 ft deep end.

• Average depth: (3.5 + 8) ÷ 2 = 5.75 ft, which is 69 inches
• Cubic feet: 32 × 16 × 5.75 = 2,944
• Gallons: 2,944 × 7.48 = 22,021
• Water at $0.01 per gallon: $220.21

Type 69 in the depth box for the second pool. The average depth is the number that matters, since one wrong guess on depth moves the answer more than any other input.

Averaging a pool with a flat shallow end

An even slope is the easy case. Many pools have a flat shallow section, a slope, and a flat deep section. Weight each part by its length.

Take a 40 ft × 20 ft pool.

• The first 10 ft is flat at 3 ft deep.
• The next 20 ft slopes from 3 ft to 8 ft, which averages 5.5 ft.
• The last 10 ft is flat at 8 ft deep.

The weighted average is (10 × 3 + 20 × 5.5 + 10 × 8) ÷ 40 = 220 ÷ 40 = 5.5 ft. In inches that is 66. The volume is 40 × 20 × 5.5 × 7.48 = 32,912 gallons.

Sample pool volumes

Pool size	Average depth	Gallons
12 × 24 ft	4 ft	8,617
15 × 30 ft	4 ft	13,464
16 × 32 ft	5 ft	19,149
20 × 40 ft	5.5 ft	32,912

These use the same formula as the calculator, with 7.48 gallons per cubic foot. A pool’s actual capacity is a little less than this, because the water level sits a few inches below the top of the wall, at the skimmer.

Round and oval pools

This calculator uses length × width, so it does not handle curves directly. For a round pool, the area is 0.7854 × diameter². A 15 ft round pool at 4 ft deep is 176.7 sq ft × 4 ft = 706.9 cubic feet, or about 5,287 gallons. For an oval pool, multiply length × width × 0.7854 for the area, then follow the same steps. If you need a very close number for chemical dosing, ask the pool builder for the stated volume.

How to measure

• Measure the water surface, not the outside of the coping.
• Measure the depth at the shallow end, the deep end and where the slope begins and ends. A stick with a tape or a marked pole works.
• Measure to the normal water line, which is usually about halfway up the skimmer opening.
• Write the numbers down in feet. Then convert the average depth to inches for the calculator.
• Write down the volume once you have it, and keep it where you keep your test kit.

What the volume is used for

Pool volume drives chemical dosing, pump and filter sizing, heater sizing and fill cost. Chlorine, salt, acid and other treatments are dosed per thousand gallons or per 10,000, and the right amount is on the product label. Follow the label and check with the manufacturer or a pool professional for your situation. Do not guess, because too much of a pool chemical can be a safety hazard.

For specific jobs, see the pool chlorine calculator, the pool salt calculator and the pool pump size calculator.

Common mistakes

• Entering feet in the depth box. Depth takes inches, so 5 ft is 60.
• Using the deepest depth instead of the average. In the 40 × 20 ft example above, using 8 ft instead of 5.5 ft overstates the volume by 45 percent.
• Forgetting that a rectangle formula overstates a pool with rounded corners.
• Treating the cost figure as a total cost of ownership. It is only the water.
• Using the 7.48 factor for a different unit. It is US gallons per cubic foot. Imperial gallons are bigger.

Related calculators

The pool cost calculator estimates the build price. For a cylindrical tank or a stock tank pool, see the tank volume calculator.