How to Calculate Swimming Pool Volume in Gallons

Measuring Your Swimming Pool

Measure the length and width of the pool. If it's rectangle, just measure the longest side and the shortest side of the pool. If it's a square, just measure one of the sides of the pool -- all four sides of the square will have the same length. And if your pool is round, just measure the diameter of the pool, which is the longest path that can travel through the center of the circle, from one edge to the other.

Measure the depth of your pool. This is the other dimension that is crucial to knowing the volume of your pool. If your pool has the same depth throughout -- most above ground pools are structured this way, for example -- then you'll just have to take one measurement from the top to the bottom of the pool. But if, like most pools, your pool has depth that gradually increases, then you'll have to measure the minimum depth (the shallow end) and the maximum depth (the deep end) of the pool.

Calculating the Volume of a Rectangular Pool

Find the average depth of the pool. To do this, you need to write down the measurements of the deep end and the shallow end of the pool and divide the result by two. Let's say the shallow end of the pool measures 3 feet and the deep end measures 8 feet. (3 + 8)/2 = 5.5. The average depth of the pool is 5.5 feet.

Multiply the result by the length and width of the pool. Now, just take this number and multiply it by the length and width of the pool. You're really just multiplying the area of the bottom of the pool by its depth. Let's say that the width of the pool is 20 feet and the length is 40 feet. So, 5.5 x 40 x 20 = 4,400 cubic feet.

Convert the answer to gallons. To convert the answer to gallons, simply multiply the answer by 7.48. 4,400 x 7.48 = 32, 912 gallons of water. You're all done.

Calculating the Volume of a Round Pool

Calculate the volume of the pool as if it were a cylinder. If you have a round pool that is of uniform depth, then all you have to do to find the volume is to treat it like a cylinder. To find the volume of a cylinder, all you have to do is find the area of the circular base times the height; this uses the formula V = πrh. Let's say the pool's radius is 4.5 feet and its depth (height) is 4 feet. Plug those numbers into the formula to find the volume: V = π x 4.5 x 4 = 254.46 cubic feet. To convert this to gallons, simply multiply the result by 7.48. 254.46 x 7.48 = 1903 gallons.

Use the shorthand formula for finding the volume of a round pool. For faster but slightly less accurate results, you can also plug the depth and diameter of the pool into the following formula: V = depth x diameter squared x 5.9. Let's say we're working with the same circle that has a depth of 4 and a diameter of 9 (the same as having a radius of 4.5). The volume of this pool = 4 x 9 x 9 x 5.9 = 1,911 gallons. The result is slightly larger.

Adjust the formula if your pool does not have the same depth. If your pool is shallower on one end and deeper on the other. just measure the minimum height and maximum height of the pool and divide them by 2 to get the average height, or the depth you should use in either formula. For example, if the pool has a minimum depth of 5 feet and a maximum depth of 10 feet, then you can add 5 + 10 to get 15, and divide 15 by 2 to get 7.5, the average depth of the pool. You can use this as the depth of the pool in either formula.

Calculating the Volume of a Free Form Pool

Find the average depth of the pool. Simply find the minimum depth and the maximum depth of the pool and divide the result by 2. Let's say the shallow end of the pool measures 3 feet and the deep end measures 8 feet. (3 + 8)/2 = 5.5. The average depth of the pool is 5.5 feet.

Find the average width of the pool. Now, just find the average of all the widths of the pool by adding up the different widths and dividing by the amount of widths. Let's say you're working with a pool that has three widths: 20, 15, and 20. So, add them up and divide by 3. 20 + 15 + 20 = 55. 55/3 = 18.3 feet. The average width of the pool is 18.3 feet.

Find the length of the pool. Let's say the length of the pool is 40 feet.

Multiply the average depth, the average width, and the length of the pool. So, 5.5 feet x 18.3 feet x 40 feet = 4,026 cubic feet.

Multiply the result by 7.48 to get the answer in gallons. 4,026 cubic feet x 7.48 = 30,114 gallons.