Water Tank Volume Calculator
Calculate the volume of a cylindrical water tank in gallons and litres from diameter and height.
How to use this tool
- Enter the tank's inside diameter and height in feet.
- The calculator returns volume in cubic feet, US gallons, and litres.
- For rectangular tanks, use length × width × height in ft³ and multiply by 7.48 for gallons.
Calculate the water storage capacity of any cylindrical tank in gallons and litres.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
Volume (ft³) = π × (diameter ÷ 2)2 × height
Volume (US gal) = ft³ × 7.48052
Volume (L) = ft³ × 28.3168
How it works
The calculator uses the standard cylinder volume formula — pi times the radius squared times the height — to find cubic feet, then multiplies by the exact conversion factors 7.48052 gal/ft³ and 28.3168 L/ft³ to produce US gallons and litres respectively. Litres are rounded to the nearest whole number.
This assumes a perfect right-circular cylinder (flat top and bottom, vertical walls). Tanks with domed lids, conical bottoms, or internal fittings will have slightly less usable volume than calculated; the result therefore represents the geometric gross capacity.
Worked example
- Radius = 6 ÷ 2 = 3 ft.
- Volume = π × 3² × 8 = π × 9 × 8 = 72π ≈ 226.19 ft³.
- Gallons = 226.19 × 7.48052 ≈ 1,692.05 US gal.
- Litres = 226.19 × 28.3168 ≈ 6,405 L.
Volume: 226.19 ft³; 1,692.05 US gallons; 6,405 litres
Common mistakes to avoid
- Entering diameter and height in different units (one in feet, one in inches) without converting, which produces a volume off by a factor of 12.
- Using diameter where radius is required in a manual calculation -- this tool takes diameter, but DIY formulas using pi*r^2 need the radius (diameter/2).
- Confusing US gallons with imperial (UK) gallons: 1 US gallon = 3.785 L; 1 imperial gallon = 4.546 L, a ~20% difference that matters for chemical dosing.
Key terms
- US gallon
- A unit of liquid volume equal to 231 cubic inches or approximately 3.785 litres. The conversion factor from cubic feet is 7.48052 gal/ft³.
- Cylinder volume
- Calculated as π × r² × h, where r is the radius and h is the height. Doubling the diameter quadruples the volume.
- Gross capacity
- The total geometric volume of a tank. Usable (net) capacity is lower because tanks are rarely filled to the very brim and may have internal components.
- Freeboard
- The vertical gap intentionally left between the water surface and the top of a tank to allow for thermal expansion, wave action, or overfill prevention.
- Static head
- The pressure exerted by the water column at the bottom of a tank, calculated as density × gravity × height. Relevant for structural design of large tanks.
Frequently asked questions
- How do I calculate water tank volume?
- For a cylindrical tank, use: Volume = π × (diameter/2)² × height. Multiply cubic feet by 7.48 for US gallons or by 28.32 for litres.
- How many gallons is a cubic foot?
- One cubic foot of water holds approximately 7.48 US gallons or 28.32 litres.