How to use the Water Tank Capacity Calculator
- 1. Choose the modeFind capacity from tank size, or size a tank for a number of people.
- 2. Enter the valuesInside dimensions and freeboard, or people, water per person and days of storage.
- 3. Get the capacityLitres, m³, cubic feet and gallons, or the tank dimensions you need.
Tank capacity formulas
Capacity in litres = volume in m³ × 1000. Use inside dimensions and the water depth (height minus freeboard).
- Rectangular: V = L × B × depth
- Vertical cylinder: V = π D²/4 × depth
- Horizontal cylinder part full to depth h: V = L × [R² cos⁻¹((R − h)/R) − (R − h)√(2Rh − h²)]
To size a tank: water needed = people × litres per person per day × days of storage. 135 litres per person per day is commonly used for homes with full flushing (IS 1172); check the current code and your local bye-laws.
Worked example and site tips
A rectangular tank 2 × 1.5 m with 1.5 m inside height and 0.15 m freeboard: water depth 1.35 m, V = 2 × 1.5 × 1.35 = 4.05 m³ = 4050 litres. A family of 5 at 135 lpcd for one day needs 675 litres; with a 1.5 m tall tank and 0.15 m freeboard, a square tank of about 0.71 × 0.71 m is enough.
- Freeboard of 0.15–0.3 m is common above the overflow level.
- Overhead tanks are often sized for about one day's need, sumps for more; check local rules.
- Wall thickness is not part of the capacity, so measure inside.
Frequently asked questions
How do I convert tank size to litres?
Work out the volume in cubic metres and multiply by 1000. For example 1 × 1 × 1 m = 1000 litres.
What is freeboard?
The empty space between the water level and the top of the tank. It stops overflow and allows for the inlet and overflow pipes.
How much water does one person need per day?
135 litres per day is commonly used for houses with full flushing (IS 1172). Your building or local authority may use a different value, so it is editable.
Is this calculator free?
Yes. You fill a short form once to see your result, and other Uncodemy tools then open without the form. Everything is calculated in your browser and nothing you type is uploaded.