Water Tank Capacity Calculator
Work out a water tank's capacity in litres, fill time and a rough pump size
| Capacity | Diameter | Height |
|---|---|---|
| 500 L | 90–92 cm | 97–101 cm |
| 1,000 L | about 110 cm | 117–126 cm |
| 2,000 L | about 135 cm | 162–164 cm |
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Results are for general guidance. Please check anything important before relying on it. Terms of use
How to Use
- Choose the tank shape: rectangular (most sumps and concrete tanks), round standing (most plastic overhead tanks) or round lying (horizontal tanks).
- Pick the unit and enter the inside measurements. The capacity in litres, kilolitres and gallons updates as you type.
- Move the water level slider to see how much water is in the tank now and how much space is left.
- Enter the filling rate in litres per minute to see how long the tank takes to fill from empty and to top up from the current level.
- Switch on Estimate the pump size and enter the total head for a rough motor size in HP.
How much water does your tank hold?
This water tank capacity calculator works out how many litres a tank holds from its inside measurements, for rectangular sumps and concrete tanks as well as round plastic or steel tanks standing up or lying on their side. You can measure in centimetres, metres, feet or inches. It also shows how much water is in the tank at the current level, how long it takes to fill at your pump's or tap's flow rate, and a rough pump size.
The formulas
Round, standing: V = π × (diameter ÷ 2)² × height
Round, lying: V = π × (diameter ÷ 2)² × length
1 m³ = 1,000 litres · 1 cubic foot = 28.317 litres
A lying round tank that is partly full is trickier, because the middle of the circle holds more water than the top and bottom. The calculator uses the area of the circular segment below the water line for that, so 25% of the depth in a lying tank is only about 20% of the volume.
Worked example: an underground sump
A sump measures 6 ft long, 4 ft wide and 5 ft deep inside. Volume = 6 × 4 × 5 = 120 cubic feet. Multiply by 28.317 and you get about 3,398 litres, or 3.4 kilolitres. Water weighs about 1 kg a litre, so that's around 3.4 tonnes when full, worth knowing for a tank on a roof or a terrace slab. If the borewell motor delivers 20 litres a minute, filling the empty sump takes 3,398 ÷ 20 = 170 minutes, about 2 h 50 min.
Typical overhead tank sizes
Round plastic overhead tanks are sold by rated capacity. Dealer listings for a major brand's models give roughly these outside sizes (checked October 2026; each model is a little different):
| Capacity | Diameter | Height |
|---|---|---|
| 500 litres | 90–92 cm | 97–101 cm |
| 1,000 litres | about 110 cm | 117–126 cm |
| 2,000 litres | about 135 cm | 162–164 cm |
If you put those outside sizes into the calculator you get more than the rated capacity, because the tanks have domed tops, tapered sides and thick walls. Use inside measurements up to the overflow pipe for the water you can actually store.
What size tank do you need?
Indian water supply planning (BIS code IS 1172 and the CPHEEO manual) uses about 135 litres per person per day for homes in towns with sewerage. A family of four therefore uses around 540 litres a day, so a 1,000-litre overhead tank lasts close to two days if the supply is cut. Where water comes on alternate days, size the sump for at least two days of use.
Rough pump size
The total head is the height from the water level in the sump or bore to the tank inlet, plus a few metres for friction in the pipes and bends. For 20 litres a minute to a tank 12 m up at 50% efficiency: 20 × 12 ÷ (4500 × 0.5) ≈ 0.11 HP of water work. That's an estimate, not a buying guide. Domestic pumps come in steps like 0.5 HP and 1 HP and their flow drops as the head rises, so check the maker's flow-versus-head chart before you buy.