Tank Volume Calculator

Sizing a tank — for fuel, water, a fish tank or a rain barrel — starts with its volume. Choose the tank shape, enter the dimensions in inches, and get the full capacity in US gallons, liters and cubic feet.

Results

Assumptions: Dimensions are internal measurements in inches, and the result is the brim-full geometric capacity.

Formula

Volume depends on the shape:
Cylinder: π × (d/2)² × length · Box: length × width × height · Sphere: 4/3 × π × (d/2)³
Cubic inches convert to US gallons by dividing by 231.

How to use this calculator

  1. Choose the tank shape.
  2. Enter the dimensions that shape needs, in inches.
  3. Press Calculate for gallons, liters and cubic feet.
Tank Volume Calculator — how the calculation works, illustrated
Tank Volume Calculator — how the calculation works, illustrated

Worked example

A cylindrical tank 48 inches across and 72 inches long.

  1. Radius: 48 ÷ 2 = 24 in.
  2. Volume: π × 24² × 72 ≈ 130,291 cubic inches.
  3. Gallons: 130,291 ÷ 231 ≈ 564.

Result: The tank holds about 564 US gallons when completely full.

Frequently asked questions

Why is usable capacity less than the calculated volume?

Tanks are seldom filled to the brim — fittings, expansion space and pickup tubes all reduce usable volume. Treat the result as the geometric maximum.

How many cubic inches are in a gallon?

Exactly 231 cubic inches make one US liquid gallon. One cubic foot is 1,728 cubic inches, or about 7.48 gallons.

Does this work for a horizontal cylinder tank?

The full capacity is the same either way — orientation only changes how volume relates to fill height, which this calculator does not model.

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