How to Calculate Water Tank Capacity in Litres (m³ and Cubic Feet)
Work out tank capacity in litres for rectangular and round tanks from metres, cm or feet. 1 m³ = 1,000 litres and 1 cubic foot = 28.317 litres, with examples.
The short answer
To find a water tank's capacity in litres, multiply its inside length × width × height in metres to get cubic metres, then multiply by 1,000 (1 m³ = 1,000 litres exactly). If you measured in feet, multiply length × width × height to get cubic feet, then multiply by 28.317 (1 ft³ = 28.316846592 litres). A tank measuring 2 m × 1.5 m × 1 m holds 3,000 litres. Use the Volume Converter to turn cubic metres or cubic centimetres into litres or gallons, and the Length Converter if your measurements are in feet or inches.
Formulas for the common tank shapes
| Tank shape | Volume formula | Then convert |
|---|---|---|
| Rectangular (sump, concrete tank, overhead box) | length × width × height | m³ × 1,000 = litres |
| Vertical cylinder (most plastic overhead tanks) | 3.1416 × radius × radius × height | m³ × 1,000 = litres |
| Horizontal cylinder, full | 3.1416 × radius × radius × length | m³ × 1,000 = litres |
The radius is half the diameter. Always use inside measurements. Concrete and brick walls take up real space, so outside dimensions overstate the capacity.
Rectangular tank: step by step in metres
Example: an underground sump, inside 2 m long, 1.5 m wide and 1 m deep.
To check it with the converter, open the Volume Converter, enter 3, set From to Cubic meter (m³) and choose Liter (L) as the result. The list of all conversions underneath also shows the same volume in US and UK gallons.
Measured in centimetres? Multiply the three numbers in cm to get cubic centimetres, then divide by 1,000. A tank 120 cm × 100 cm × 100 cm is 1,200,000 cm³, which is 1,200 litres. In the converter, choose Cubic centimeter (cm³) as the input unit.
Rectangular tank measured in feet
Many sumps and tanks in India are built and quoted in feet.
Example: inside 6 ft × 4 ft × 4 ft.
The Volume Converter does not have a cubic-foot unit, so there are two ways to use the tools:
- Multiply by 28.317 yourself after finding the cubic feet, as above.
- Convert each measurement to metres first with the Length Converter (6 ft = 1.8288 m, 4 ft = 1.2192 m), multiply them to get 2.7184 m³, then convert to litres. Both routes give 2,718.4 litres.
A larger example: a 10 ft × 8 ft × 5 ft sump is 400 ft³, which is about 11,327 litres.
Cylindrical tank: step by step
Example: a vertical plastic tank with an inside diameter of 1.1 m and a water height of 1.3 m.
In feet: a tank 4 ft in diameter and 5 ft tall is 3.1416 × 2 × 2 × 5 = 62.83 ft³, which is about 1,779 litres.
Plastic tanks are sold by nominal capacity (500, 1,000, 2,000 litres and so on). The usable volume is usually a little lower, because the outlet sits above the bottom and the overflow sits below the top. Measure the height between those two points if you want the volume you can actually use.
Quick reference
| Unit | Litres |
|---|---|
| 1 cubic metre (m³) | 1,000 L |
| 1 cubic foot (ft³) | 28.317 L |
| 1,000 cubic centimetres (cm³) | 1 L |
| 1 US gallon | 3.785 L |
| 1 UK (imperial) gallon | 4.546 L |
And the other way round: 1,000 litres is 1 m³, about 35.31 ft³, about 264.2 US gallons or about 220 UK gallons.
Working with real tanks
- Measure the water height, not the tank height, if you want to know how much water is in it now. For a rectangular tank the water volume is length × width × water depth.
- Sloping or uneven floors. Measure the depth at several points and use the average.
- Tanks with rounded corners or tapered sides hold a little less than the box formula suggests. Treat the result as a maximum.
- Daily use. Divide the capacity by your daily water use to see how many days a full tank lasts. A 2,000-litre tank for a household using 500 litres a day lasts about four days.
- Ordering a water tanker. Tankers are usually sold by litres. Knowing your sump's capacity stops you from paying for more than it can hold.
Common mistakes
- Using outside dimensions. Measure inside the walls.
- Mixing units. Do not multiply a length in feet by a width in metres. Convert everything to the same unit first.
- Using the diameter instead of the radius in the cylinder formula. That makes the answer four times too big.
- Forgetting the 1,000. 3 m³ is 3,000 litres, not 3 litres.
- Using 28 or 30 litres per cubic foot. 28.317 is the accurate figure. Using 30 overstates a 400 ft³ sump by about 673 litres.
Related tools and guides
- Converting between US and UK gallons and litres: US Gallon vs UK Gallon
- Converting plot and floor area for the space around your tank: Area Converter
- Feet and inches to centimetres for measuring: Length Converter
Frequently asked questions
How do I calculate a tank's capacity in litres?
Multiply inside length x width x height in metres to get cubic metres, then multiply by 1,000. A 2 m x 1.5 m x 1 m tank holds 3,000 litres.
How many litres are in a cubic foot?
One cubic foot is 28.316846592 litres, so about 28.32 litres.
How many litres is a 6 x 4 x 4 ft tank?
6 x 4 x 4 = 96 cubic feet, and 96 x 28.3168 = about 2,718 litres.
How do I find the capacity of a round tank?
Use 3.1416 x radius x radius x height. A tank 1.1 m across and 1.3 m high holds about 1.235 m3, or 1,235 litres.
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