How many litres of water is that?
A litre of water weighs a kilogram almost exactly — the definition the kilogram started life with. That makes it the easiest weight to hold and remember.
20 × a litre of water
20 kg ÷ 1 kg = 20 bottles.
Reference figures are measurements and are rounded; the unit conversion behind them is exact.
| Reference | Size | Yours ÷ theirs | Note |
|---|---|---|---|
| half-litre bottle | 0.5 kg | 40× | |
| litre of water | 1 kg | 20× | |
| two-litre bottle | 2 kg | 10× | |
| five-litre jug | 5 kg | 4× |
Worth knowing
- The kilogram was originally defined as the mass of a litre of water at its densest, 3.98 °C. The modern definition uses the Planck constant, but the coincidence still holds to about 0.003%.
- A person is roughly 60% water, so a 75 kg adult carries about 45 litres of it.
Frequently asked
Does a litre of water always weigh a kilogram?
Very nearly. At 4 °C it is 1.000 kg; at 25 °C, 0.997 kg. Salt water is about 2.5% heavier.
How heavy is a gallon of water?
A US gallon is 3.785 litres, so 3.79 kg or 8.34 lb. An imperial gallon is 4.546 litres, 4.55 kg or 10 lb — which is where the old rhyme "a pint’s a pound and a quarter" comes from.