How to Calculate Pool Volume in Gallons (Any Shape)

You need to know your pool's volume in gallons for almost every pool task: how much chemical to add, how long to run the pump, which size of pump and filter to buy, how long to fill it, and how much a heater will have to warm. The calculation is a short formula, and the only measurements you need are length, width (or diameter) and an average depth.

The basic formula

One cubic foot of water holds 7.48 US gallons. The surface area depends on the shape, and the average depth is the mean of the shallow and deep ends.

Formulas by shape

• Rectangular: length × width × average depth × 7.48
• Round: π × (diameter ÷ 2)² × average depth × 7.48, which is diameter × diameter × average depth × 5.9
• Oval: length × width × average depth × 5.9
• Irregular or kidney: divide it into rectangles and circles, work out each, and add them up, or use the "average of several measurements" approach.

The average depth for a pool with a sloping floor is (shallow depth + deep depth) ÷ 2. This is a close approximation for most pools. If the pool has a flat shallow section and a steep slope to a deep end, measure each part separately.

Worked examples

• A 16 × 32 ft rectangular pool, 3½ ft at the shallow end and 8 ft at the deep end: average depth 5.75 ft. Volume = 16 × 32 × 5.75 × 7.48 ≈ 22,000 gallons.
• A 15 ft round above-ground pool, filled to a flat 4 ft: π × 7.5² × 4 × 7.48 ≈ 5,300 gallons.
• A 12 × 24 ft rectangular pool with a 4 ft average depth: 12 × 24 × 4 × 7.48 ≈ 8,600 gallons.

Round pools by size

Approximate gallons for round above-ground pools filled to a flat depth, as measured at the water level:

Above-ground pools are not usually filled to the very top, and their label depth (such as 52 inches) is the wall height. Measure the actual water depth.

Rectangular pools by size

Or let the pool volume calculator do it. It also estimates the time to fill with a hose.

Why the number matters

• Chemicals. Doses on product labels are per number of gallons. Under-dosing leaves the water unsafe and cloudy, and over-dosing wastes money and can irritate skin and eyes, and damage the liner and equipment. Always follow the label, and test the water before and after adding anything.
• Pump and filter size. A common guideline is that the pump should turn over the whole pool's volume in 8 to 10 hours. Knowing the volume tells you the minimum flow rate.
• Heating. It takes a lot of energy to warm thousands of gallons, so size the heater to the volume.
• Cost to fill. Water is priced per thousand gallons, so you can estimate the bill before you start.

Filling time

A garden hose delivers about 5 to 10 gallons per minute. At 9 gallons per minute, a 5,300 gallon pool takes about 10 hours, and a 22,000 gallon pool takes about 41 hours. To check your own hose, time how long it takes to fill a 5 gallon bucket, and divide. If you are filling from a well, check that the well can keep up, and be prepared to run the hose for days.

Measuring tips

• Measure to the water level, not to the top edge of the wall.
• For an oval or a pool with rounded corners, measure at the widest and longest points.
• For a pool with a deep end and a slope, take several depth readings and average them. A weighted tape or a pole with marks is handy.
• Write down the answer, so you do not need to work it out each season.