Swimming Pools and Spas: How to Calculate Volume (in Gallons)

Calculating the volume of swimming pools and spas can be a vital task, since most water treatment chemicals and cleaners require an estimate to provide an effective dose. Figuring out the number of gallons in swimming pools and spas may seem a bit complicated, but simple measurements and formulas make the job a snap.

Determine the shape of the pool or hot tub. The shape will determine which measurements you’ll take and which formula you’ll use.

Square/Rectangular Swimming Pools and Spas

Square or rectangular swimming pools and spas consist of four sides and four corners that meet at right angles. To calculate the volume in gallons for square or rectangular swimming pools and spas:

Measure the length and width in feet.
Calculate the average depth by adding the depth of the deep end to the depth of the shallow end and dividing by two.
Multiply length by width, and then multiply by average depth.
Multiply that value by 7.5 to get the capacity in gallons.

Round Swimming Pools and Spas

Round swimming pools and spas are perfectly circular in shape. To calculate the volume in gallons for round swimming pools and spas:

Measure the diameter in feet.
Calculate the average depth by adding the depth of the deep end to the depth of the shallow end and dividing by two.
Square the diameter (multiply it by itself), and then multiply by average depth. Multiply that value by 5.9 to find the capacity in gallons.

Elliptical Swimming Pools and Spas

Elliptical swimming pools and spas are shaped like elongated circles. To calculate the volume in gallons for elliptical swimming pools and spas:

Measure the length (long diameter) and width (short diameter) in feet.
Calculate the average depth by adding the depth of the deep end to the depth of the shallow end and dividing by two.
Multiply the length by the width, and then multiply by average depth. Multiply that value by 5.9 to find the capacity in gallons.

Oval Swimming Pools and Spas

Oval swimming pools and spas are shaped like rectangles with rounded ends. To calculate the volume in gallons for oval swimming pools and spas:

Measure the overall length and width in feet.
Calculate the average depth by adding the depth of the deep end to the depth of the shallow end and dividing by two.
Multiply the overall length by the width, and then multiply by the average depth. Multiply that value by 6.7 to find the capacity in gallons.

Kidney-Shaped Swimming Pools and Spas

Kidney-shaped swimming pools and spas are shaped like an oval with an inward curve on one side. To calculate the volume in gallons for kidney-shaped swimming pools and spas:

Measure the length in feet.
Calculate the average width by measuring the widest point at each end, adding those values together, and dividing by two.
Calculate the average depth by adding the depth of the deep end to the depth of the shallow end and dividing by two.
Multiply the length by the average width, and then multiply by the average depth. Multiply that value by 7.0 to find the capacity in gallons.

Free-Form Swimming Pools and Spas

Free-form swimming pools and spas are basically those that don’t fall into any of the previous shape categories. To calculate the volume in gallons for free-form swimming pools and spas:

Estimate the average length and width by taking several measurements in feet, adding them together, and then dividing the number of measurements you took. The more you measure, the more accurate your estimate will be.
Calculate the average depth by adding the depth of the deep end to the depth of the shallow end and dividing by two.
Multiply the average length by the average width, and then multiply by the average depth. Multiply that value by 7.5 to find the capacity in gallons.

By following these simple steps, you should have no problem coming up with a volume estimate for any swimming pools or spas, regardless of their shape.