Water Weight Calculator
Work out how much a volume of water weighs – from litres, gallons or the size of a tank, pond or butt.
Result
Enter a volume or tank size.
How it works
Weight = volume × density. Fresh water has a density of 1,000 kg per cubic metre (1 kg per litre) at 4 °C, slightly less when warmer, and sea water about 1,025 kg/m³ because of the dissolved salt. For a box-shaped tank the volume is length × width × depth; for a round one it’s π × (diameter ÷ 2)² × depth.
- 1 litre ≈ 1 kg
- 1 UK gallon ≈ 4.55 kg (10 lb)
- 1 US gallon ≈ 3.79 kg (8.35 lb)
- 1 cubic metre = 1,000 litres ≈ 1 tonne
- A 200-litre water butt ≈ 200 kg when full
Frequently asked questions
How much does a litre of water weigh?
Almost exactly 1 kg. That was the original definition of the kilogram. Warm water weighs very slightly less: about 998 g per litre at 20 °C.
How much does a gallon of water weigh?
A UK gallon of water weighs about 4.55 kg (10 lb) – which is how the imperial gallon was defined. A US gallon is smaller, about 3.79 kg (8.3 lb).
Can my floor take a fish tank or hot tub?
Water is heavy – a 200-litre tank holds 200 kg of water before the tank, gravel and stand. For large tanks, hot tubs or upstairs installations, ask a structural engineer or the manufacturer.
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