Multiply length x width x water depth in feet to get cubic feet, then multiply by 28.32 for litres or 6.23 for gallons. An 8 x 6 ft tank with 5 ft of water holds 240 cft, which is 6,796 litres or about 1,495 gallons.
Use "Tank capacity" for a tank you have or plan, or "Size for my home" to find the storage your family needs.
How to calculate water tank capacity
Rectangular or cube tank
Capacity = length x width x depth of water. In feet, 1 cft = 28.317 litres = 6.229 gallons. In metres, 1 cubic metre = 1,000 litres.
Example: 8 x 6 ft, water 5 ft deep: 8 x 6 x 5 = 240 cft; 240 x 28.317 = 6,796 litres; 6,796 / 4.546 = 1,495 gallons.
Round (cylinder) tank
Capacity = 3.1416 x radius x radius x height, or 0.7854 x diameter² x height.
Example: a tank 4 ft across with 5 ft of water: 0.7854 x 16 x 5 = 62.83 cft = 1,779 litres or 391 gallons.
Measure inside the walls. For an underground or roof RCC tank, use the depth up to the overflow pipe.
Capacity of common RCC tank sizes
| Inside size (L x W x depth) | Cubic feet | Litres | Gallons |
|---|---|---|---|
| 4 x 4 x 4 ft | 64 | 1,812 | 399 |
| 6 x 4 x 4 ft | 96 | 2,718 | 598 |
| 8 x 5 x 4 ft | 160 | 4,531 | 997 |
| 8 x 6 x 5 ft | 240 | 6,796 | 1,495 |
| 10 x 6 x 5 ft | 300 | 8,495 | 1,869 |
| 10 x 8 x 6 ft | 480 | 13,592 | 2,990 |
| 12 x 10 x 6 ft | 720 | 20,388 | 4,485 |
The gallon in Pakistan is the imperial gallon of 4.546 litres. Tank makers and water boards use it, so 1,000 litres is about 220 gallons. A US gallon is smaller, 3.785 litres.
What size tank does a house need?
Pakistan's National Drinking Water Policy 2009 treats 120 litres per person per day as adequate urban supply. Actual use in big cities is higher: a 2023 study put demand at 60 gallons a person a day in Islamabad and 46 in Rawalpindi WASA areas, and the Karachi water board has planned on 54 gallons.
Example: 6 people x 120 litres = 720 litres a day, about 158 gallons. A roof tank filled every day should hold at least that, so a 200 gallon tank. If water comes only every other day, store two days: 1,440 litres, about 317 gallons, so a 350 gallon or larger tank.
Many houses keep an underground tank that stores several days of supply and pump it up to a smaller roof tank. Size the underground tank from the gap between supply days, and the roof tank for one day.
Plastic tanks are sold by capacity in gallons or litres. Before you buy, check the tank's height and diameter against the space on the roof and the strength of the slab: a full 500 gallon tank weighs more than two tons.
Water tank questions
How many litres are in 1 cubic foot?
28.317 litres, which is 6.229 imperial gallons. So multiply a tank's volume in cft by 28.317 for litres. One cubic metre is 1,000 litres or 35.31 cft.
How many litres are in a 1,000 gallon tank?
4,546 litres, using the imperial gallon of 4.546 litres that Pakistani tank makers use. In a rectangular RCC tank that is about 160.5 cft, for example 8 x 5 x 4 ft.
How do I calculate water in a partly full tank?
Measure the depth of the water, not the tank, and use that as the height. For a round tank, 0.7854 x diameter squared x water depth gives the volume. For a horizontal round tank the formula is different and this calculator does not cover it.
How do I convert litres to gallons?
Divide litres by 4.546 for imperial gallons, the gallon used for tanks in Pakistan. So 1,000 litres is about 220 gallons and 2,000 litres about 440 gallons. To go from gallons to litres, multiply by 4.546. For US gallons use 3.785 instead.
How much does the water in a tank weigh?
One litre of water weighs about 1 kg. A 500 gallon tank holds about 2,273 litres, so about 2,273 kg of water, more than two tons. Roof tanks need a slab or stand designed for that weight.
Last reviewed 30 September 2026. Results are estimates; official notices always take precedence.