Water Tank Size Calculator (OHT and Underground)
How the formula works
A water tank must store enough water to meet the daily demand of the building, even when the supply is interrupted. Most buildings use an underground tank (UGT) that receives water from the municipal line or a borewell, and an overhead tank (OHT) that feeds the building by gravity. This calculator sizes both tanks from the number of occupants, and it can also find the capacity of an existing tank from its dimensions.
Formulas
Daily water demand (L) = Number of occupants × Water use per person per day (L)
OHT capacity = Daily demand × OHT storage fraction
Underground tank capacity = Daily demand × Storage days + Fire reserve
Total storage = OHT capacity + Underground tank capacity
Tank capacity from dimensions
Rectangular tank: Volume (m³) = Length × Width × Water height
Cylindrical tank: Volume (m³) = π ÷ 4 × Diameter² × Water height
Capacity in litres = Volume in m³ × 1000
1 m³ = 1000 litres = 264.17 US gallons
Where
- Water use per person per day = typical consumption for the building type, in litres
- OHT storage fraction = share of the daily demand kept in the overhead tank (commonly one third to one half)
- Storage days = number of days of demand kept in the underground tank (commonly 1 to 2 days)
- Fire reserve = water kept for fire fighting, if the project requires it in the underground tank
- Water height = height of the water, not the full height of the tank
Typical water use values
- Residential flats or houses: about 135 litres per person per day
- Residential premium or villa: about 200 litres per person per day
- Office: about 45 litres per person per day
- School (day): about 45 litres per student per day
- Hospital: about 340 litres per bed per day
- Hotel: about 180 litres per guest per day
- Restaurant: about 70 litres per seat per day
- Factory: about 45 litres per worker per day
- Cinema or theatre: about 15 litres per seat per day
These are typical design values, similar to those given in the National Building Code of India. Your local code and project specification always have the final say. Choose Custom in the calculator if you know the exact value for your project.
How to use the calculator
- Select the calculation type.
- For tank size, select the building type, enter the number of occupants, and select the tank type.
- Select the OHT storage and the underground storage days that apply. Add a fire reserve if your project needs it.
- Enter the water depth if you also want the plan size of a square tank.
- For capacity from dimensions, select the shape and the unit, then enter the dimensions and the water height.
- Click Calculate. You can then download the result as a PDF.
Important notes
- Add free board (about 0.3 m) above the water depth when you fix the tank height.
- Domestic water, flushing water and fire water may be stored separately in some projects. Follow your project specification.
- The result is a planning estimate. The final size must be checked against the local code and the water supply conditions.
Worked example
Example 1: Residential building with OHT and underground tank
A residential building has 100 occupants. Use 135 litres per person per day, OHT storage of one third of the daily demand, and 1 day of storage in the underground tank. Use a water depth of 2 m.
Step 1: Daily demand
100 × 135 = 13,500 L per day (13.50 m³)
Step 2: OHT capacity
13,500 × 1/3 = 4,500 L (4.50 m³)
Square plan side = √(4.50 ÷ 2) = 1.50 m, so the OHT is 1.50 m × 1.50 m × 2 m water depth
Step 3: Underground tank capacity
13,500 × 1 = 13,500 L (13.50 m³)
Square plan side = √(13.50 ÷ 2) = 2.60 m, so the tank is 2.60 m × 2.60 m × 2 m water depth
Step 4: Total storage
4,500 + 13,500 = 18,000 L
Example 2: Office building with fire reserve
An office has 500 occupants at 45 litres per person per day. The OHT stores half of the daily demand, the underground tank stores 1.5 days of demand, and the project needs a 50,000 L fire reserve in the underground tank.
Daily demand = 500 × 45 = 22,500 L
OHT = 22,500 × 0.5 = 11,250 L
Underground tank = 22,500 × 1.5 + 50,000 = 83,750 L
Total storage = 11,250 + 83,750 = 95,000 L
Example 3: Capacity of a rectangular tank
A rectangular tank is 3 m long and 2 m wide, and the water height is 1.5 m.
Volume = 3 × 2 × 1.5 = 9 m³ = 9,000 L
In US gallons = 9,000 × 0.264172 = about 2,378 gallons
Base area = 3 × 2 = 6 m²
Example 4: Capacity of a cylindrical tank
A round tank has a diameter of 1.2 m and a water height of 1.5 m.
Volume = π ÷ 4 × 1.2² × 1.5 = 1.70 m³, which is about 1,696 L (about 448 US gallons).
Common mistakes
- Using the full tank height instead of the water height. The usable water height is lower than the tank height because of the free board and the outlet level.
- Using one water use value for every building. A hospital, a hotel and an office use very different amounts of water per person. Select the right type or use your project value.
- Counting only the number of flats. Use the number of people. For flats, multiply the flats by the people per flat.
- Making the OHT too small or too large. The OHT is usually a part of the daily demand, not the whole demand. A very big OHT adds heavy load on the structure.
- Forgetting the fire reserve. If your fire fighting design needs stored water, add it separately, so that the domestic water is not used up by it.
- Mixing units. Check that length, width and height are in the same unit. 1 m³ is 1000 litres, not 100.
- Ignoring the supply conditions. If the supply hours are short or irregular, the underground tank needs more storage days.
- Skipping the local code. The typical values here are only a guide. Always check the code that applies to your project.
Frequently asked questions
How do I calculate the size of a water tank?
Multiply the number of occupants by the water use per person per day to get the daily demand. Then multiply it by the storage you want in the tank. For example, 100 people × 135 litres = 13,500 litres per day.
How much water does one person need per day?
For residential buildings, a typical design value is about 135 litres per person per day. Offices use about 45 litres, hotels about 180 litres, and hospitals about 340 litres per bed. Your local code may use different values.
What is the size of an OHT for a house or building?
The OHT is commonly sized for one third to one half of the daily demand. For a 100 person residential building (13,500 litres per day), that is about 4,500 to 6,750 litres.
What is the size of an underground water tank?
It is commonly sized for 1 to 2 days of the daily demand, plus the fire reserve if the project needs it. For 13,500 litres per day, a 1 day tank is 13,500 litres.
What is the difference between an OHT and an underground tank?
The underground tank stores water received from the supply, and a pump lifts it to the overhead tank. The overhead tank then feeds the building by gravity. Together they keep water available when the supply stops or the power is off.
How do I find the capacity of a rectangular tank in litres?
Multiply the length, width and water height in metres to get cubic metres, then multiply by 1000. A tank of 3 m × 2 m × 1.5 m holds 9 m³, which is 9,000 litres.
How do I find the capacity of a round (cylindrical) tank?
Use π ÷ 4 × diameter² × water height, in metres, then multiply by 1000 for litres. A tank with a 1.2 m diameter and 1.5 m water height holds about 1,696 litres.
How many US gallons are in a litre?
1 litre is about 0.2642 US gallons, so 1000 litres is about 264 US gallons. The calculator gives the result in US gallons for the capacity calculation.
Should I add free board to the tank?
Yes. Add about 0.3 m of free board above the water level when you fix the tank height. The calculator uses only the water depth.
Does this calculator replace a plumbing design?
No. It is a planning aid. The final tank size must follow the local code, the supply conditions and the project specification.