Water Tank Volume Calculator
Knowing how much a tank actually holds matters whether you are sizing rainwater storage, planning irrigation or working out how long a supply will last. This calculator finds the capacity of a cylindrical water tank from two measurements you can take with a tape: the diameter across the top and the height of the wall. It applies the standard cylinder volume formula, pi times the radius squared times the height, where the radius is simply half the diameter, and then converts that geometric volume into US gallons using the exact factor of 7.48052 gallons per cubic foot. The result is the brim-full capacity, the most the tank could ever hold to the very top. In practice usable volume is a little lower, because real tanks keep an air gap, sit an overflow below the rim and lose a little space to inlet and outlet fittings, so treat the figure as a maximum and trim it for your working fill level. For accuracy, measure the inside dimensions, since the tank wall does not hold water. Every figure here is computed deterministically from the geometry you enter, and the worked example below reconciles exactly to the calculator.
A cylindrical tank holds V = pi x r squared x h, where r is half the diameter. A tank 4 ft across and 5 ft tall holds about 62.83 cubic feet, which is roughly 470.01 US gallons brim full.
Water tank volume formula
V = pi x r^2 x h
gallons = V x 7.48052
r = radius = diameter / 2
h = inside height
V = volume in cubic feet
The cross-section of a round tank is a circle of area pi times the radius squared. Multiplying by the height stacks that circle into a volume, and the gallon factor converts cubic feet into US liquid gallons.
Worked example
A round tank is 4 feet in diameter and 5 feet tall.
- Radius: 4 / 2 = 2 feet
- Volume: pi x 2^2 x 5 = pi x 4 x 5 = 62.83 cubic feet
- Gallons: 62.83 x 7.48052 = 470.01 US gallons
The tank holds about 470.01 US gallons brim full. These are the calculator's default inputs, so the result above matches the widget exactly.
Capacity of common round tanks
Approximate brim-full capacity in US gallons for a few sizes.
| Diameter | Height | US gallons |
|---|---|---|
| 3 ft | 4 ft | 211.51 |
| 4 ft | 5 ft | 470.01 |
| 5 ft | 6 ft | 881.32 |
| 6 ft | 8 ft | 1,692.18 |
Measurement and unit guidance: US Food and Drug Administration (FDA).
Water tank volume calculator: frequently asked questions
How do I calculate the volume of a cylindrical water tank?
Use the cylinder volume formula: volume equals pi times the radius squared times the height. The radius is half the diameter. For a tank 4 feet across and 5 feet tall, the radius is 2 feet, so the volume is pi times 2 squared times 5, which is about 62.83 cubic feet.
How do I convert cubic feet to gallons?
One cubic foot holds 7.48052 US gallons. Multiply the tank volume in cubic feet by 7.48052 to get US gallons. So 62.83 cubic feet is about 470.01 US gallons. For imperial UK gallons, multiply cubic feet by 6.22884 instead.
Does the tank ever hold its full calculated volume?
Rarely in practice. The calculated figure is the geometric capacity to the very top. Real tanks have an air gap, an overflow below the rim and fittings that take up space, so usable volume is a little lower. Treat the result as the maximum and reduce it for the working fill level you actually use.
What if my tank is rectangular, not cylindrical?
For a rectangular tank, volume is length times width times height, all in the same unit. Then convert to gallons the same way, multiplying cubic feet by 7.48052. This calculator uses the cylinder formula because most water storage tanks are round, but the gallon conversion is identical.
Should I measure inside or outside dimensions?
Measure inside dimensions for capacity. The wall thickness of the tank does not hold water, so using outside measurements overstates the volume slightly. For a thin-walled poly tank the difference is small, but for thick concrete or steel tanks it can matter.
Official sources
- Consumer measurement and product guidance: US Food and Drug Administration (FDA). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.