Rainfall to volume calculator.

Worked example

25 mm of rain over 100 m² is 0.025 m × 100 m² = 2.5 m³ = 2,500 litres (660 US gallons). One inch (25.4 mm) over one acre (4,046.873 m²) is 0.0254 × 4,046.873 = 102.79 m³ = 102,791 litres, or 27,154 US gallons.

How it works

Volume = depth × area. The calculator converts depth to metres and area to square metres to get cubic metres, then multiplies by 1,000 for litres and divides by 3.785412 for US gallons. Conversion factors: 1 in = 0.0254 m, 1 ft² = 0.09290304 m², 1 acre = 4,046.873 m², 1 US gallon = 3.785412 L, 1 hectare = 10,000 m².

This is the rain that falls on the area. What a roof delivers to a tank is less, because of first flush, splash, evaporation and overflow; use the rainwater roof yield calculator for that. Gallons here are US gallons, not imperial gallons.

Sources and basis (all opened 2026-10-08): NIST Guide for the Use of the SI, Appendix B conversion factors (inch, foot, square foot, acre, US gallon), and the USGS Water Science School rainfall calculator (rain depth over an area gives a volume of water).

FAQ

How many gallons is 1 inch of rain on an acre?

About 27,154 US gallons, which is about 102,791 litres.

How many litres is 1 mm of rain?

One litre per square metre. 10 mm over 100 m2 is 1,000 litres.

Does all the rain on my roof reach the downspout or tank?

No. Some is lost to first flush, splash, evaporation and overflow. This tool gives the rain that falls, not what you can collect.

Are these US or imperial gallons?

US gallons. One US gallon is 3.785412 litres.

What area should I use for a pitched roof?

The footprint, meaning the area covered when seen from above, not the sloped surface.