Water Tank Calculator
Work out a water tank's capacity — rectangular or cylindrical — in litres, US gallons and cubic metres, with the usable volume at your fill level.
A tank is a box or a cylinder — and its capacity is the first number in any water system. This calculator is that number, with the usable volume at your real fill level.
Enter the dimensions; the capacity follows in litres, gallons and cubic metres.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Choose the tank shape.
- Enter the dimensions.
- Set the fill level.
- Read the capacity and the usable volume.
- Use the Copy results button to copy the answer.
Worked example
| Tank | 2 m × 1 m × 1 m |
|---|---|
| Capacity | 2 × 1 × 1 = 2 m³ |
| Litres | 2 × 1,000 = 2,000 L |
| US gallons | 2,000 ÷ 3.785411784 = 528.34 gal |
| At 50% fill | 1,000 L usable |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
How do I calculate a water tank's capacity?
Multiply length × width × height for a rectangular tank, or π × (diameter ÷ 2)² × height for a cylindrical one. This calculator does both, in litres, gallons and cubic metres.
What is the usable volume?
The capacity at your fill level — a 2,000-litre tank filled to 50% holds 1,000 usable litres. Set the fill level to match how full you actually keep it.
How big a tank do I need?
Work out your daily use with the hot water usage calculator, then size the tank for a day or two of use — plus a margin. This page gives you the capacity; the usage page gives you the demand.
Is this a substitute for plumbing design?
No. This is a capacity calculation for planning. Tank sizing, pressure and code compliance depend on local regulations and real installation conditions — see the safety notice.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.