Partly filled tanks handled exactly
Water Tank Capacity CalculatorLitres, Gallons & the Right Tank Size for Your Home
Find out exactly how much water your tank holds. Pick the tank shape, enter its size in feet, inches, meters or centimeters, and this water tank capacity calculator gives the capacity in litres, gallons and cubic units, plus how much water is in it right now. It also sizes the overhead tank and sump for your family and works out how long your pump takes to fill it.
- 3 tank shapes
- 4 length units
- Live fill gauge
- 3 tools in one
Use inside measurements. If you only know the radius, double it to get the diameter.
135 litres per person per day matches the common Indian guideline for homes with full flushing. Set sump days to 0 if you have no sump.
Time a bucket of known size under the inlet to measure your real flow rate.
How to calculate water tank capacity
Tank capacity is simply the inside volume of the tank. Work out the volume for your shape, then convert it to litres. One cubic meter holds 1,000 litres, and one cubic foot holds about 28.32 litres.
Rectangular tank
V = length × width × heightVertical round tank
V = π × (diameter ÷ 2)² × heightHorizontal round tank
V = π × (diameter ÷ 2)² × lengthPartly filled, horizontal
area = r²·acos((r−h)/r) − (r−h)·√(2rh−h²)Litres from cubic feet
litres = cft × 28.317Litres from cubic meters
litres = m³ × 1,000Example: rectangular sump
A sump 6 ft long, 4 ft wide and 4 ft deep holds 6 × 4 × 4 = 96 cft. That is 96 × 28.317, or about 2,718 litres.
Example: round overhead tank
A tank 4 ft across and 5 ft tall holds π × 2² × 5 ≈ 62.83 cft, which is about 1,779 litres.
Which water tank size does my family need?
A common planning figure for Indian city homes is 135 litres per person per day. The table shows how many people each standard tank size serves for one day at that rate. In practice, add a margin for guests and dry days.
| Tank size | People for 1 day | Weight when full | Cubic feet |
|---|---|---|---|
| 500 litres | 3 | ~500 kg | 17.7 |
| 750 litres | 5 | ~750 kg | 26.5 |
| 1,000 litres | 7 | ~1 tonne | 35.3 |
| 1,500 litres | 11 | ~1.5 tonnes | 53.0 |
| 2,000 litres | 14 | ~2 tonnes | 70.6 |
| 3,000 litres | 22 | ~3 tonnes | 105.9 |
| 5,000 litres | 37 | ~5 tonnes | 176.6 |
Weight excludes the tank itself. A full overhead tank puts a heavy load on the roof, so check slab strength before installing a large one.
Volume unit conversions
| Unit | In litres |
|---|---|
| 1 cubic meter | 1,000 |
| 1 cubic foot | 28.317 |
| 1 US gallon | 3.785 |
| 1 UK (imperial) gallon | 4.546 |
| 1 cubic inch | 0.0164 |
More than a tank volume calculator
Most tank calculators stop at total volume. This one also tells you how much water is in the tank now, what size to buy, and how long filling will take.
Live fill gauge
Slide the water level and watch the tank drawing and litres update together.
Exact horizontal tanks
Partly filled horizontal tanks use the true circular-segment formula, not a straight percentage.
Family tank sizing
Get the overhead tank to buy and a sump size with suggested dimensions for your household.
Pump fill time
See how long the pump runs to fill the tank and how many days the water will last.
Weight and roof load
Know the full weight and the load per square meter before placing a tank on your roof.
Every unit
Measure in feet, inches, meters or centimeters and read results in litres, gallons, m³ and cft.
How to use the water tank capacity calculator
Choose the shape
Pick rectangular, vertical round or horizontal round to match your tank.
Enter the size
Type the inside length, width, height or diameter in the unit you measured.
Set the level
Slide the water level to see how many litres are in the tank right now.
Plan further
Size a tank for your family or check how long your pump needs to fill it.
Building a new sump? Estimate the walls with the brick calculator and the base slab with the concrete calculator.
Benefits and water tank tips
Buy the right size
Avoid tanks that run dry every evening or sit half empty and grow algae.
Leave a freeboard
The top few inches above the overflow pipe never fill, so usable capacity is a little lower than total.
Check roof strength
Every 1,000 litres weighs about a tonne. Place large tanks over beams or columns.
Clean regularly
Clean and disinfect tanks at least twice a year, and keep lids tightly closed.
Stop overflow
A float valve or automatic pump controller prevents overflow and saves power.
Know pump costs
Estimate the running cost of your pump with the electricity cost calculator.
Water tank capacity FAQs
How do I calculate water tank capacity in litres?
Find the inside volume of the tank, then convert it. For a rectangular tank, multiply length, width and height. If you measured in feet, multiply the cubic feet by 28.317 to get litres. If you measured in meters, multiply the cubic meters by 1,000.
How many litres are in 1 cubic foot?
One cubic foot holds about 28.32 litres of water. For example, a tank with a volume of 100 cubic feet holds roughly 2,832 litres.
What size water tank does a family of 4 need?
At about 135 litres per person per day, a family of four uses around 540 litres daily. A 750 or 1,000 litre overhead tank covers a full day with a margin, and a sump of about 1,500 litres covers two more days.
How do I calculate the capacity of a cylindrical tank?
Multiply pi (3.1416) by the radius squared and by the height or length. The radius is half the diameter. Then convert the result to litres using 28.317 per cubic foot or 1,000 per cubic meter.
How do I find the water in a partly filled horizontal tank?
A horizontal round tank is not filled in a straight line, because it is narrower near the top and bottom. The water's cross-section is a circular segment, which this calculator works out exactly from the water depth.
How long will my pump take to fill the tank?
Divide the litres needed by the flow rate. For example, filling 800 litres at 40 litres per minute takes 20 minutes. Real flow drops as the pump lifts water higher, so measure your actual flow with a bucket for accuracy.
How heavy is a full water tank?
One litre of water weighs about 1 kilogram, so a full 1,000 litre tank weighs roughly a tonne plus the weight of the tank. Make sure the roof or stand can safely carry this load.
Should I use inside or outside measurements?
Always use inside measurements, because the walls take up space. For concrete sumps the difference can be large, and the space above the overflow pipe also reduces the usable capacity.
Know every litre in your tank
Whether you are buying a new overhead tank, building a sump or just checking the water level, this water tank capacity calculator gives you clear answers in seconds. Size it right, fill it smart and never run dry.