Weather

Rain Convert Calculator

Convert a rainfall depth between millimeters and inches. Choose a direction, then enter the depth.

Free to use — no sign-up or login.

Measured rain depth. Zero is allowed. 1 in = 25.4 mm.

Converted rainfall

This rain converter turns a precipitation depth between millimeters and inches with the international inch definition 1 in = 25.4 mm. Choose millimeters to inches or inches to millimeters, then enter the rainfall depth. The result is a unit conversion only — not a forecast or storm total.

For a general multi-family tool, use the Unit Converter. Nearby outdoor planning pages include the Dew Point Calculator and Hiking ETA Calculator.

Millimeters and inches for rainfall

Weather services often report rain in millimeters (metric) or inches (U.S. customary). Because 1 inch is defined as exactly 25.4 millimeters, mm → in divides by 25.4 and in → mm multiplies by 25.4. Direction is a menu limited to those two choices; displayed results may round even though the factor is exact.

What this tool does not do

It does not estimate rain rate, storm duration, runoff, or flood risk. Enter a depth you already have (from a gauge, radar product, or report) and read the equivalent in the other unit.

Worked example: 25.4 mm to inches

Choose millimeters to inches and enter 25.4. Inches = 25.4 / 25.4 = 1.0 in. Inches to millimeters with a depth of 1 returns 25.4 mm.

\[ 25.4\,\mathrm{mm}/25.4=1\,\mathrm{in} \]

Frequently asked questions

How do I choose millimeters or inches?

Use the Convert menu. Millimeters to inches divides by 25.4. Inches to millimeters multiplies by 25.4. Those are the only two directions.

Is 1 inch exactly 25.4 mm?

Yes. The international inch is defined as exactly 25.4 mm, so the conversion is exact aside from display rounding.

Can I convert rain rate (mm/h) here?

Only if you treat the number as a depth in mm or inches. The tool does not convert rates or time units by itself.

References