How to Calculate Gallons in a Round Tank

Written by irene blake
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How to Calculate Gallons in a Round Tank
Depending on its size, you might need to use a ladder to measure the tank's depth. (ladder shadow image by Jim Mills from

High school mathematics will help you calculate the number of gallons a round tank holds. Just use a simple formula based on the tank's depth and radius. Measure the inside dimensions to calculate the capacity most accurately.

Skill level:

Things you need

  • Measuring tape
  • Calculator
  • Ladder

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  1. 1

    Calculate the radius of the tank. The radius equals the tank's diameter divided by two. (The diameter is the distance from a point on one side of the tank to the point directly opposite.) That means if the diameter is 22 feet, the radius is 11 feet (because 22 divided by 2 equals 11). Convert any partial foot to a decimal. For example, a diameter of 17 feet has a radius of 8.5 feet.

  2. 2

    Multiply the radius by itself to obtain its square. (For example, if the radius is 11 feet, multiply it by 11 to get 121.)

  3. 3

    Multiply 3.1417 (pi) by the radius' square. For example, 3.1417 times 121 equals 380.146.

  4. 4

    Measure the depth. Convert any partial foot to a decimal. Depending on the tank's size, you might need a ladder to stand on.

  5. 5

    Multiply the result in Step 3 by the depth of the tank to convert the measurements and calculate the tank's capacity in cubic feet. (For example, 380.146 multiplied by a depth of 10 feet gives you 3801.46 cubic feet.)

  6. 6

    Multiply the result in Step 5 by 7.47. This converts the measurement into gallons. For example, 3801.46 times 7.47 gives you 28,397 gallons (rounded off). This is the water capacity in gallons of a 22-foot-by-10-foot-round tank.

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