The most common units for measuring rainfall are:

* Millimeters (mm): This is the standard unit in many parts of the world.

* Inches (in): Used in the United States and some other countries.

To convert between millimeters and inches:

* 1 inch = 25.4 millimeters

* 1 millimeter = 0.03937 inches

Therefore, you need to provide the measurement in inches or another unit to convert it to millimeters.