Rainfall Converter
Convert rainfall between mm, inches, liters per sq meter, and gallons per sq foot.
Also calculates total water collected from a roof for any storm.
Type in any field — the others update instantly.
Rainfall measures how much water falls on a surface.
Key relationship:
- 1 mm of rain = 1 liter per square meter (exact)
- This is because 1 mm depth × 1 m² area = 0.001 m³ = 1 liter
Conversions:
- 1 inch of rain = 25.4 mm (exact)
- 1 inch of rain = 25.4 liters per m²
- 1 mm of rain = 0.03937 inches
- 1 gallon per sqft = 40.746 liters per m² = 40.746 mm
Practical references:
| Description | mm | inches |
|---|---|---|
| Light drizzle | 0.1–2.5 mm | 0–0.1" |
| Light rain | 2.5–7.5 mm | 0.1–0.3" |
| Moderate rain | 7.5–50 mm | 0.3–2" |
| Heavy rain | 50–100 mm | 2–4" |
| Very heavy rain | 100+ mm | 4+" |
Annual rainfall examples:
| Location | Annual (mm) | Annual (in) |
|---|---|---|
| London | ~600 mm | ~24" |
| New York | ~1,270 mm | ~50" |
| Sydney | ~1,215 mm | ~48" |
| Mumbai | ~2,400 mm | ~95" |
The reason rainfall is reported as a depth rather than a volume is that depth is independent of area, so the same “20 mm” describes the storm whether it fell on a flowerpot or a whole county. The neat consequence is the 1 mm equals 1 liter per square meter identity, which turns roof-harvest math into simple multiplication: roof footprint in square meters times rainfall in millimeters gives liters collected, before losses.
Those numbers add up faster than people expect. A modest 100 square meter roof in a 10 mm shower sheds about 1,000 liters, which is why even a light rain fills a water butt quickly and why poor drainage floods so readily. Real collection runs a bit below the theoretical figure, since some water evaporates, splashes off, or stays behind wetting the surface, so a runoff efficiency around 0.8 is a sensible planning factor. And note that a gauge measures the horizontal catch only; wind-driven rain hitting a wall never shows up in the official total even though it soaks the building all the same.
How we build and check this converter
This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.