What this calculator does
Tank capacity is simply the volume in cubic metres multiplied by a thousand. An 8 by 4 metre pool averaging 1.5 metres deep holds 48,000 litres.
The weight is the figure worth pausing on. That water weighs 48 tonnes, because a cubic metre of water is a tonne. Anything holding water needs a base that can carry it, which is why a full 1,000 litre tank cannot simply be placed on a timber deck without checking what the deck can support.
The formula
For a rectangular tank the volume is length times width times depth. For a round one it is pi times the radius squared times the depth, with the first dimension read as the diameter. Multiplying cubic metres by 1,000 gives litres, and water weighs one tonne per cubic metre.
| Term | Meaning |
|---|---|
| Cubic metre | Equal to 1,000 litres, and to one tonne of water. |
| Average depth | For a pool with a sloping floor, the mean of the shallow and deep ends. |
| Freeboard | The gap between the water surface and the tank top, so nominal capacity exceeds usable volume. |
| US gallon | 3.785 litres. The imperial gallon is 4.546, so the two differ by about 20%. |
The inputs explained
| Field | What to enter |
|---|---|
| Shape | Rectangular or round. For a round tank the length field is read as the diameter. |
| Length (or diameter) (m) | Length in metres, or diameter for a round tank. |
| Width (rectangular only) (m) | Width in metres. Ignored for a round tank. |
| Average depth (m) | Average depth. For a sloping pool floor, use the mean of the two ends. |
When to use it
Sizing a rainwater tank
Matching tank capacity to roof catchment and expected demand.
Dosing a pool
Chemical additions are specified per volume, so the capacity has to be known.
Checking what a structure must support
Water is heavy, and the weight figure is what determines whether a base is adequate.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does depth change the capacity?
The same footprint at a range of depths.
| Average depth | Capacity | Weight of water | Time to fill at 20 L/min |
|---|---|---|---|
| 1 m | 32,000 litres | 32.00 tonnes | 26.7 hours |
| 1.5 m | 48,000 litres | 48.00 tonnes | 40.0 hours |
| 2 m | 64,000 litres | 64.00 tonnes | 53.3 hours |
| 3 m | 96,000 litres | 96.00 tonnes | 80.0 hours |
Questions
How much does water weigh?
One tonne per cubic metre, or one kilogram per litre. This makes the weight trivial to read off the volume, and it is the figure that matters when working out whether a stand, deck or slab can carry a full tank.
How do I handle a pool with a sloping floor?
Use the average of the shallow and deep end depths, which is accurate for a floor sloping uniformly. For a pool with a distinct deep end and a flat shallow section, calculate the two parts separately and add them.
Is the stated tank capacity the usable volume?
Usually not quite. Tanks are rated at nominal capacity but are filled to below the top, leaving freeboard, and the outlet often sits above the very bottom. Usable volume is typically a few percent less than the rating.
Which gallon does this use?
US gallons, at 3.785 litres each. The imperial gallon used in the UK is 4.546 litres, about 20% larger, so a capacity quoted in gallons means noticeably different things either side of the Atlantic.
For pipe volumes, see the pipe volume calculator. For a well instead, see the well volume calculator.