How much water is in that rainfall?
One millimetre of rain is one litre per square metre. That single equivalence makes every other rainfall figure computable.
2 × 10 mm on a 50 m² roof
1,000 L is bigger than 4 of the 7 references on this page.
Reference figures are measurements and are rounded; the unit conversion behind them is exact.
| Reference | Size | Yours ÷ theirs | Note |
|---|---|---|---|
| 1 mm on a square metre | 1 L | 1,000× | |
| 1 mm on a 50 m² roof | 50 L | 20× | |
| water butt | 210 L | 4.76× | |
| 10 mm on a 50 m² roof | 500 L | 2× | One decent shower |
| a wet UK year on a 50 m² roof | 4,500 L | 0.222× | About 900 mm |
| 1 mm on a football pitch | 7,140 L | 0.14× | |
| 10 mm on a hectare | 100,000 L | 1/100 |
Worth knowing
- One millimetre of rain on one hectare is 10 000 litres — ten cubic metres, or ten tonnes of water.
- A 50 m² roof in a place with 900 mm of annual rainfall sheds 45 000 litres a year, most of which goes straight to the drain.
Frequently asked
How much water is 1 mm of rain?
Exactly one litre per square metre. That is what the millimetre measures — the depth the water would stand at if none drained away.
How big a water butt do I need?
A 200-litre butt fills from just 4 mm of rain on a 50 m² roof, so it overflows constantly. Sizing for summer drought rather than for rainfall is the usual mistake.