How to Calculate the Gallon Capacity of an Area

Finding the gallon capacity of a specific area can come in handy when you are trying to find out many gallons of water your fish tank or swimming pool will require. This may sound like a daunting task, but it is simple to execute if you have the appropriate equation. Even if you've forgotten most of your high school geometry, all you have to do is take the measurements and plug them into the formula below.

Boy (6-8) using protractor, focus on pot of pencils, close up
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Step 1

If the area is rectangular, take your tape measure and find out the length and width of the area. If the area is circular, take your tape measure and find the radius by measuring across the middle and dividing that number in half. Write these numbers down so you don't forget them.

Step 2

Multiply the length by the width of your area for a rectangle. For a circle, multiply the radius by itself (radius x radius), then multiply that number by 3.14, also known as pi.

Step 3

Measure the depth of the area. If the area has varying heights, such as a swimming pool with a deep end and a shallow end, find the average depth by adding the height of the deepest and the shallowest ends, then dividing that number in half.

Step 4

Multiply the area you calculated in step 2 by the depth (or average depth), then multiply that number by 7.48, regardless of whether you are working with a circle or rectangle. There are 7.48 gallons in a cubic foot.