Overhead Water Tank Calculator
Find the capacity in litres of an overhead water tank from its size, plus the load it puts on the slab.
Inputs
Formula
- Volume (cu ft) = length × width × usable depth
- Capacity (litres) = volume × 28.317
- 1 m³ = 1000 litres = 35.31 cu ft
A rectangular tank holds length × width × depth. Water is never filled to the very top, so subtract a free space (freeboard) of about 0.5 ft for an overhead tank and 1 ft for a sump. One cubic foot holds 28.317 litres, which is the only conversion you need.
Step-by-step calculation
- 1
Usable depth
depth − freeboard
5.5 ft
- 2
Volume
L × W × usable depth
264 cu ft
- 3
Capacity
cu ft × 28.317
7,476 litres
Worked example
- 1A sump 8 ft × 6 ft × 6 ft with 1 ft freeboard has a usable depth of 5 ft.
- 2Volume = 8 × 6 × 5 = 240 cu ft.
- 3Capacity = 240 × 28.317 ≈ 6,796 litres.
Tips
- Design the depth first — deeper tanks use less slab area and less concrete.
- Add an air vent and a manhole of at least 600 mm for cleaning.
Frequently asked questions
How many litres in one cubic foot?
28.317 litres. So 35.31 cu ft makes exactly 1,000 litres.
Why leave free space at the top?
For the inlet, the float valve and to stop overflow splashing. About 0.5–1 ft is normal.
Can I use metres instead of feet?
Enter your dimensions in feet — 1 metre is 3.28 feet, so a 3 m long tank is 9.84 ft.
Related calculators
Keep going with more construction tools.
Water Tank Size Calculator
Find the right water tank size for your home from the number of people, daily usage and how many days of storage you want.
Septic Tank Size Calculator
Find the right septic tank size for a home from the number of users and desludging interval, with suggested dimensions.
Steel Weight Calculator
Calculate the weight of steel bars, pipes, plates, flats and angles from dimensions using the standard 7850 kg/m³ density and the D²/162 rebar rule.
Concrete Calculator
Work out concrete volume for slabs, beams, columns and footings, with cement bags, sand and aggregate quantities for any nominal mix.
Brick Calculator
Estimate the number of bricks, mortar volume, cement bags and sand needed for any wall from its area and thickness.
Cement Calculator
Cement, sand and water quantities for plastering, flooring screed and masonry mortar — in bags, kilograms and cubic metres.