Water Tank Capacity Calculator
Capacity of square and round water tanks in litres and gallons — and how long to fill.
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About the Water Tank Capacity Calculator
Planning an underground or overhead tank, or checking how much water yours holds? Enter the inside dimensions of a rectangular or round tank to get the capacity in litres, imperial gallons (the gallon usually meant in Pakistan) and US gallons.
Add your pump's flow rate to see how long it takes to fill.
How to use the Water Tank Capacity Calculator
- Choose the tank shape and units.
- Enter the inside length, width or diameter, and the water height.
- Optionally enter the pump flow in litres per minute.
- Click Calculate capacity.
Formula
Rectangular: volume = length × width × height. Standing round tank: volume = π × (diameter ÷ 2)² × height. Lying round tank: area of the water segment × length.
1 m³ = 1,000 litres = 35.31 cu ft; 1 imperial gallon = 4.546 litres; 1 US gallon = 3.785 litres.
Examples
- A tank 8 × 6 ft, 5 ft deep → 240 cu ft = 6,796 litres (1,495 imperial gallons). A 30 L/min pump fills it in about 3 h 47 min.
- A round tank 4 ft across, 5 ft high → 1,779 litres.
Frequently asked questions
Should I measure inside or outside?
Measure inside — walls and lids take up space. For an overhead plastic tank, the litre size on the label is the usual capacity.
How accurate is this estimate?
It is a planning estimate based on standard formulas. Actual quantities depend on workmanship, materials and site conditions, so confirm with your contractor before buying.