Rainfall Converter: mm, Inches and Litres
One millimetre of rain over one square metre is exactly one litre. One inch of rain is 25.4 mm, and over one acre it is about 27,154 US gallons (102,790 litres).
Total water collected
1,000 litres
That is 1 m³ or 264 US gallons.
Millimetres
10.00
Inches
0.394
Area
100.0 m²
Show the math
- 1
Convert the depth to millimetres
10 mm
One inch is exactly 25.4 mm, and one centimetre is 10 mm.
= 10 mm = 0.394 in
Read the steps as text
- Convert the depth to millimetres. 10 mm One inch is exactly 25.4 mm, and one centimetre is 10 mm.
- Convert the area to square metres. 100 m²
- Multiply depth by area to get litres. 10 mm × 100 m² = 1,000 litres One millimetre of rain over one square metre is exactly one litre, so millimetres times square metres gives litres.
- Convert litres to other volumes. 1,000 L ÷ 1,000 = 1 m³ · 1,000 L ÷ 3.7854 = 264 US gallons
What rainfall depth means
Rainfall is reported as the depth of water that would collect on flat ground if none drained away, evaporated or soaked in. A rain gauge measures exactly that. 10 mm of rain means the water would be 10 mm deep, whatever the size of the area.
To convert units, one inch is exactly 25.4 mm and one centimetre is 10 mm. A quarter inch is about 6.4 mm and a tenth of an inch is about 2.5 mm. In the US rainfall is usually in inches, elsewhere in millimetres.
From depth to volume
Multiply the depth by the area to get a volume. Because 1 mm over 1 m² equals 1 litre, the calculation is simply millimetres × square metres = litres. A 100 m² roof in a 10 mm shower receives 1,000 litres. The calculator converts other area units for you (square feet, acres and hectares) and shows the answer in litres, cubic metres and US gallons.
For rainwater harvesting, multiply the result by a collection efficiency of about 0.8 to 0.9 to allow for evaporation, splashing and first flush losses. Gutters, roof material and filters all affect what you actually capture.
How heavy is the rain?
Forecasters also rate the rate of rainfall, in depth per hour. Light rain is under about 2.5 mm an hour (0.1 in), moderate rain is about 2.5 to 7.6 mm an hour, and heavy rain is more than that. A depth on its own, such as 50 mm, could be a slow drizzle over two days or a violent storm over an hour, so the time it fell in matters for flooding.
Frequently asked questions
- How many litres is 1 mm of rain?
- One litre for every square metre. So 1 mm of rain over 50 m² is 50 litres.
- How do I convert inches of rain to mm?
- Multiply the inches by 25.4. Two inches of rain is 50.8 mm.
- How much water can I collect from my roof?
- Roof area in m² × rainfall in mm gives litres landing on the roof. Multiply by about 0.8 to 0.9 for what you can realistically collect.
- What does 1 inch of rain over an acre weigh?
- About 102,790 litres, which is roughly 102.8 tonnes, or around 27,154 US gallons.