Most of the world measures rainfall in millimeters; the US uses inches. 1 inch of rain equals exactly 25.4 mm — the same international inch defined in NIST SP 811. When comparing weather forecasts, irrigation schedules, or plant care instructions across countries, this converter lets you move between units instantly.
The formula
inches = mm ÷ 25.4
mm = inches × 25.4
The factor 25.4 is exact — it is the international definition of one inch, not an approximation.
Practical examples
Example 1: A UK weather forecast shows 18 mm of rain this week. How many inches is that? 18 ÷ 25.4 = 0.71 inches
Example 2: A US drip irrigation guide recommends 1.5 inches per week. How many mm should I program into my metric controller? 1.5 × 25.4 = 38.1 mm
Example 3: A plant tag says it needs 600 mm of annual rainfall. Your area averages 24 inches. Is it enough? 24 × 25.4 = 609.6 mm — yes, just enough
Common mistakes
Confusing rain depth with water volume. Rainfall depth (e.g., 1 inch) describes how many inches of water would accumulate on a flat surface — it doesn't directly tell you gallons. To convert to volume, multiply depth × area: 1 inch over 1,000 sq ft ≈ 623 gallons.
Assuming rain gauge readings in garden centers are always metric. US rain gauges typically display in inches with a 1/100 inch scale. EU and Commonwealth gauges use mm. Read the scale carefully before converting.
Rounding 25.4 to 25. The correct factor is exactly 25.4, not 25. Over a typical monthly rainfall reading of 100 mm, rounding introduces a 1.6% error (100/25 = 4.0 in vs. 100/25.4 = 3.94 in).
International and regional variations
| Country / Region | Standard unit | Typical annual range |
|---|---|---|
| United States | Inches | 8–60 in (desert to Pacific Northwest) |
| United Kingdom | Millimeters | 600–1,600 mm/year |
| Australia | Millimeters | 200–2,000+ mm/year |
| Canada | Millimeters | 300–1,500 mm/year |
Quick reference
| Millimeters (mm) | Inches (in) | Gardening context |
|---|---|---|
| 6.4 | 0.25 | Light rain shower |
| 12.7 | 0.5 | Half the weekly target for most lawns |
| 25.4 | 1.0 | Weekly benchmark for most vegetables |
| 50.8 | 2.0 | Heavy weekly rain; watch for overwatering |
| 100 | 3.94 | Very heavy week; typical for tropical climates |
| 254 | 10.0 | Typical monthly total in rainy season |