Water tank capacity calculator: litres, sump and overhead tank size
This water tank capacity calculator does two jobs. First, it turns the measurements of a rectangular tank, an upright round tank or a horizontal cylinder into litres, cubic metres and US and UK gallons, working in metres, feet, centimetres or inches, with an optional water depth for a part-full tank. Second, it sizes the overhead tank and sump a household needs from the number of people, litres per person per day and days of storage.
It is meant for house owners, builders and plumbers who want a quick, checkable number before ordering a tank or drawing the plan. Everything runs in your browser; nothing is uploaded.
How to use the Water Tank Capacity Calculator
- For capacity, choose the tank shape and unit, then enter the inside dimensions. Add a water depth if the tank is not full.
- Read the litres, cubic metres, cubic feet and gallons, and the weight of the water.
- For a family, switch to Size for a family and enter the people and litres per person per day (135 is pre-filled).
- Set the days of storage, the share kept overhead, and any fire reserve for the sump.
- Read the overhead and sump volumes, the next common tank size and the footprint at your chosen water depth.
Water Tank Capacity Calculator features
Three tank shapes
Rectangular, upright cylinder and horizontal cylinder, the last with an exact part-full calculation.
Feet or metres
Change the unit and the numbers you typed convert, so the tank stays the same size.
Family sizing
Daily demand = people × litres per head, multiplied by the days you want to store.
Overhead and sump split
You set the ratio. The tool gives each volume, the nearest larger stock size and a footprint for a chosen water depth.
Gallons both ways
US gallons (3.785 L) and UK gallons (4.546 L) side by side.
Formulas for tank volume
Use inside measurements, since the wall thickness of concrete or plastic reduces the water space. 1 m³ is 1,000 litres and 1 ft³ is 28.32 litres. For any other solid, the volume calculator works in the same way.
| Tank | Volume | Litres |
|---|---|---|
| Rectangular | length × width × height | × 1,000 if in metres |
| Upright cylinder | π × (diameter ÷ 2)² × height | × 1,000 if in metres |
| Horizontal cylinder, full | π × (diameter ÷ 2)² × length | × 1,000 if in metres |
| Horizontal, part full | segment area × length | segment from the depth of water |
How much water does a family need?
India’s code of basic requirements for water supply, IS 1172:1993, and the CPHEEO Manual on Water Supply and Treatment (1999) are the sources usually cited for a domestic design figure of 135 litres per person per day (lpcd), which CPHEEO applies to towns with full flushing. Published readings of the IS code give other values by town size and housing type, so the calculator lets you change it. CPHEEO splits the 135 L roughly as 55 L for bathing, 30 L for flushing and 20 L for laundry, with 10 L each for drinking and cooking, washing up and cleaning.
Worked example: 4 people × 135 lpcd = 540 L a day. For two days of storage that is 1,080 L; with 40 % overhead the overhead tank is 432 L (next stock size 500 L) and the sump is 648 L (750 L). At 1 m of water depth the overhead tank needs a footprint of 0.43 m², about 0.66 m square.
Overhead tank vs sump
- The sump (underground tank) stores the supply that arrives when the mains or tanker runs, and feeds the pump. The overhead tank gives pressure by gravity and a buffer for a few hours or a day.
- The split is a rule of thumb: keeping about one third to one half of the storage overhead is common. Heavier overhead tanks need a stronger slab and columns, which is a structural decision.
- Fire storage, if your building needs it, is set by the National Building Code and the local fire officer. Enter the volume as a reserve and it is added to the sump.
- Add the volume of pipes and allow freeboard above the overflow. To size the pump to lift water to the overhead tank, see the water pump HP calculator.
Water Tank Capacity Calculator FAQ
How do I calculate the capacity of a water tank in litres?
Multiply the inside length, width and height in metres to get cubic metres, then multiply by 1,000. A 1.5 × 1 × 1 m tank is 1.5 m³, or 1,500 litres.
How do I find the litres in a round tank?
Use π × radius² × height. A tank 1.2 m across and 1.5 m high holds π × 0.6² × 1.5 = 1.696 m³, or about 1,696 litres.
How big an overhead tank do I need for a family of four?
At 135 litres per person per day the family uses 540 litres daily. Overhead tanks are commonly sized from half a day to a day or more of demand; the calculator splits your total storage between overhead tank and sump by the ratio you set.
What is lpcd?
Litres per capita per day, the water used per person per day. 135 lpcd is the common domestic design basis for Indian towns; confirm the figure for your area.
How do I size a sump?
Work out the daily demand, choose the days of storage you want, and take the share not kept overhead. Add any fire reserve, then pick the next stock size or plan the dimensions from the footprint shown.
Is the result exact?
The geometry is exact for the numbers entered. Real tanks have curved corners, sloping bases and a usable level below the overflow, so treat it as a planning figure.