How To Calculate Volume Of Water In Any Swimming Pool

In order to figure out the proper doses of chemicals for your pool, you need to determine how many gallons of water it holds. To do that, you need to use a ruler to measure your pool to find out these four different numbers; Length, Width, Average depth and a multiplier that determines gallons.

So here are the formulas to figure out how much water is in your swimming pool:
These formulas hold true for both In-ground pools as well as above ground pools.

Rectangular, Square or Free Form Pools

Length x Width x Average Depth x 7.5 = Total gallons

For example: 16′ x 32′ x 4.5′ x 7.5 = 17,280 Gallons

Circular Pools

Diameter x Diameter x Average Depth x 5.9 = Total gallons

For example: 18′ x 18′ x 4′ x 5.9 = 7,646 Gallons

Oval Pools

Length x Width x Average Depth x 5.9 = Total gallons

For example: 15′ x 30′ x 4′ x 5.9 = 10,620 Gallons

How to determine the Average Depth of a Swimming Pool where the bottom slopes

Below is a side-shot of the “typical” inground pool that has a shallow end (see “D”), then a slope that leads into the deep end (see “C”).

How many gallons of water are in my pool?

First measure the depth of “D”, the shallow end (usually anywhere from 3′ to 5′), and then measure the depth of “C”, the deep end (usually anywhere between 6′ and 12′) . Then, add them together and divide by 2. This is the average depth of your pool.

For example, if your shallow end “D” is 3′, and your deep end “C” is 8′, this would be your formula for the Average Depth:

3′ + 8′ = 11′

11′ divided by 2 = 5.5.

Therefore, in this example, your average depth would be 5 1/2 feet.

Kidney and Irregular Shaped Swimming Pools

There are two methods used to calculate the capacity of irregular shapes. First, you can imagine the pool or hot tub as a combination of smaller, regular shapes. Measure these various areas and use the calculations described previously for each square or rectangular area and for each circular area. Add these volumes together to determine the total capacity.

The total of measurement A plus measurement B multiplied by 0.45 multiplied by the length gives you the surface area of the kidney shape. (A + B = 18 feet). The rest of the calculations you are now familiar with. Try this volume calculation:

0.45 x (A+B) x length x average depth x 7.5 = volume (in gallons)

0.45 x 18 ft x 25 ft x 5 ft x 7.5 = 7593.75 gallons.

Rectangular In-Ground Pool Volumes in Gallons

Pool Size
(in feet)
3.5 ft
Avg Depth
4 ft
Avg Depth
4.5 ft
Avg Depth
5 ft
Avg Depth
All Answers Are In Gallons

Round And Oval Above Ground Pool Volumes In Gallons

Size48in Wall*52in Wall*
12ft Round2,975 3,398
15ft Round4,646 5,310
18ft Round7,646 8,602
21ft Round9,106 10,408
24ft Round11,895 13,594
27ft Round15,054 17,205
30ft Round18,585 21,240
33ft Round22,488 25,700
12x24ft Oval5,948 6,797
15x30ft Oval9,293 10,620
16x32ft Oval10,573 12,084
18x33ft Oval12,267 14,019
Assuming water depth is 6 inches less than wall height.
All Answers Are In Gallons

What Is Parts Per Million (ppm)

One of the most important calculation you will use is parts per million (ppm). The amount of solids and liquids in the water is measured in parts per million, as in three parts of chlorine in every one million parts of water, or 3 ppm.

To help, this list shows common terms and their equivalents:

Square foot (sq. ft.) = 12 inch wide x 12 inch long

Cubic foot (cu. ft.) = 12 inch wide x 12 inch long x 12 inch high

Cubic yard (cu. yd.) = 36 inch wide x 36 inch long x 36 inch high

One cubic foot of water contains 7.48 gallons

One cubic foot of water weighs 62.4 pounds

One gallon of water weighs 8.33 pounds

One part per million (ppm) represents 8.3 pounds of chemical per million gallons of water.

However, one gallon of chlorine, for example, poured into one million gallons of water does not equal 1 ppm. That is because the two liquids are not of equal density. This becomes obvious since a gallon of water weighs 8.3 pounds but a gallon of chlorine weighs 10 pounds (in a 15 percent solution). The chlorine is a denser liquid; there’s more of it than an equal volume of water.