OneTaskBy Tim King · Updated
Water Tank Capacity Calculator
Cylinder, Horizontal and Rectangular Tanks in Litres and US or UK Gallons
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How the Water Tank Capacity Calculator works
This water tank capacity calculator works out how much a tank holds when full and how much water is in it at a given depth. Choose a vertical cylinder, a horizontal cylinder or a rectangular tank, enter its size in metres or feet, and it gives the volume in litres, US gallons and UK gallons, plus the percentage full.
Use the inside dimensions of the tank. For water butts, rainwater tanks and oil tanks the quoted capacity is often the brim-full volume; the useful volume is less, because the outlet sits above the base and the overflow below the top.
How the water tank capacity calculator works
The formula for a vertical cylinder is π × (d ÷ 2)² × h, and for a box it is length × width × height. For a horizontal cylinder partly full to depth h with radius r, the wetted area is r² × arccos((r − h) ÷ r) − (r − h) × √(2rh − h²), multiplied by the length. For example, a 1.2 m diameter tank 1.5 m tall holds 1,696 litres, and 1,131 litres (66.7%) when filled to 1 m. A second example: a 4 ft diameter horizontal tank 8 ft long holds 752 US gallons, and exactly half at 2 ft deep.
A horizontal tank is not linear: the first and last few centimetres hold much less than the middle, so a dipstick reading cannot be turned into how many litres remain by simple proportion. That is why the depth input matters.
Capacity of a 1 m tall vertical cylinder tank by diameter
| Diameter | Litres | US gallons | Imperial gallons |
|---|---|---|---|
| 0.5 m | 196 | 52 | 43 |
| 0.6 m | 283 | 75 | 62 |
| 0.7 m | 385 | 102 | 85 |
| 0.8 m | 503 | 133 | 111 |
| 0.9 m | 636 | 168 | 140 |
| 1.0 m | 785 | 207 | 173 |
| 1.1 m | 950 | 251 | 209 |
| 1.2 m | 1,131 | 299 | 249 |
| 1.3 m | 1,327 | 351 | 292 |
| 1.4 m | 1,539 | 407 | 339 |
| 1.5 m | 1,767 | 467 | 389 |
| 1.6 m | 2,011 | 531 | 442 |
| 1.7 m | 2,270 | 600 | 499 |
| 1.8 m | 2,545 | 672 | 560 |
| 1.9 m | 2,835 | 749 | 624 |
| 2.0 m | 3,142 | 830 | 691 |
| 2.1 m | 3,464 | 915 | 762 |
| 2.2 m | 3,801 | 1,004 | 836 |
| 2.3 m | 4,155 | 1,098 | 914 |
| 2.4 m | 4,524 | 1,195 | 995 |
| 2.5 m | 4,909 | 1,297 | 1,080 |
| 2.6 m | 5,309 | 1,403 | 1,168 |
| 2.7 m | 5,726 | 1,513 | 1,259 |
| 2.8 m | 6,158 | 1,627 | 1,354 |
| 2.9 m | 6,605 | 1,745 | 1,453 |
| 3.0 m | 7,069 | 1,867 | 1,555 |
FAQ
How do I calculate the capacity of a round tank?
Measure the inside diameter and height in metres, then work out π × (diameter ÷ 2)² × height to get cubic metres, and multiply by 1,000 for litres. A tank 1 m across and 1 m tall holds 785 litres, or about 207 US gallons.
How many litres are in a cubic metre?
There are 1,000 litres in one cubic metre. One cubic foot holds 28.3 litres, or 7.48 US gallons. So a rectangular tank 2 × 1 × 1 m holds 2,000 litres, and a 6 × 3 × 3 ft tank holds about 1,346 US gallons.
How do I work out the water left in a horizontal tank?
Measure the depth of water with a dipstick, choose horizontal cylinder and enter the depth. The calculator uses the circular segment formula, because at a quarter of the depth a horizontal tank is only about 20% full, not 25%.
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