Weather services in most countries report rain in millimeters, while the US reports it in inches. If you follow international forecasts, garden by a rain gauge, or compare rainfall between countries, you need to switch between the two. The conversion is simple: 1 inch of rain = 25.4 mm.
Rainfall MM to Inches Chart
| Rainfall (mm) | Inches | Water per square meter |
|---|---|---|
| 1 mm | 0.04 in | 1 L/m² |
| 2 mm | 0.08 in | 2 L/m² |
| 5 mm | 0.20 in | 5 L/m² |
| 10 mm | 0.39 in | 10 L/m² |
| 15 mm | 0.59 in | 15 L/m² |
| 20 mm | 0.79 in | 20 L/m² |
| 25 mm | 0.98 in | 25 L/m² |
| 25.4 mm | 1.00 in | 25.4 L/m² |
| 30 mm | 1.18 in | 30 L/m² |
| 40 mm | 1.57 in | 40 L/m² |
| 50 mm | 1.97 in | 50 L/m² |
| 75 mm | 2.95 in | 75 L/m² |
| 100 mm | 3.94 in | 100 L/m² |
| 150 mm | 5.91 in | 150 L/m² |
| 200 mm | 7.87 in | 200 L/m² |
| 250 mm | 9.84 in | 250 L/m² |
| 300 mm | 11.81 in | 300 L/m² |
Rainfall Inches to MM
| Rainfall (inches) | Millimeters |
|---|---|
| 0.1 in | 2.5 mm |
| 0.25 in | 6.4 mm |
| 0.5 in | 12.7 mm |
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127.0 mm |
What Does “1 mm of Rain” Actually Mean?
Rainfall is measured as the depth of water that would build up on flat, level ground if none of it drained away or evaporated. So 10 mm of rain means a 1 cm layer of water over the whole area.
A handy fact for gardeners and anyone collecting rainwater: 1 mm of rain equals exactly 1 liter of water on every square meter. A 20 m² roof collects about 200 liters from a 10 mm shower (before any losses). In US units, 1 inch of rain on one acre is about 27,154 US gallons.
How Heavy Is That Rain?
Rain intensity is described by how much falls per hour. The American Meteorological Society uses roughly these bands:
| Description | Millimeters per hour | Inches per hour |
|---|---|---|
| Light rain | Up to 2.5 mm/h | Up to 0.10 in/h |
| Moderate rain | 2.6 to 7.6 mm/h | 0.11 to 0.30 in/h |
| Heavy rain | More than 7.6 mm/h | More than 0.30 in/h |
Other national weather services use slightly different thresholds, so treat these as a general guide rather than an official definition everywhere.
How to Convert Rainfall Quickly
- mm to inches: divide by 25.4. Quick estimate: divide by 25. 50 mm ÷ 25 ≈ 2 in (exact: 1.97 in).
- inches to mm: multiply by 25.4. Quick estimate: multiply by 25. 3 in × 25 ≈ 75 mm (exact: 76.2 mm).
For any other value, use the mm to inches converter or the printable mm to inches chart.
Reading a Rain Gauge
- Place it in an open spot, away from roofs, walls and trees that shelter it or drip into it.
- Read it at the same time every day, looking at the water level at eye height.
- Many gauges have both mm and inch scales. On the inch side, small marks are usually 0.01 in (about 0.25 mm).
- Empty it after each reading so the next day starts from zero.
Frequently Asked Questions
How many inches is 10 mm of rain?
10 mm of rain is 0.39 inches — just under 4/10 of an inch.
Is 25 mm of rain a lot?
25 mm (about 1 inch) in a day is a significant soaking in most places. Falling in a single hour it would count as heavy rain.
How many mm is 1 inch of rain?
Exactly 25.4 mm.
Related: Popular mm to inches conversions · MM to Inches chart (printable) · How to read a tape measure