Many weather services around the world 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 (rounded) | 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 | 2.54 mm |
| 0.25 in | 6.35 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’s Glossary of Meteorology defines light, moderate and heavy rain by these hourly rates (millimeters or inches per hour, not per day):
| 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 weather services use different thresholds, and rain rates are not the same as daily totals, so treat these as a general guide rather than a universal definition.
How to Convert Rainfall Quickly
- mm to inches: divide by 25.4. Quick estimate: divide by 25. 50 mm ÷ 25 ≈ 2 in (more precisely 50 ÷ 25.4 ≈ 1.97 in, rounded to 2 decimals).
- 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 often 0.01 in (0.254 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 about 0.39 inches (0.3937 in) — just under 4/10 of an inch.
Is 25 mm of rain a lot?
25 mm is just under 1 inch (0.98 in). Over a whole day it is a substantial amount of rain in many places, and if it fell in a single hour it would count as heavy rain (the heavy rain threshold is more than 7.6 mm per hour).
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
Sources and Checking
- NIST Guide to the SI, Appendix B.8 (1 inch = 0.0254 m exactly)
- American Meteorological Society Glossary: Rain (light, moderate and heavy rain rates)
Published by Johnson, who runs mmintoinches.com. Every value in this guide was recalculated from 1 inch = 25.4 mm and checked against the sources below on 9 October 2026. Found a mistake? Email support@mmintoinches.com with the page and the value.