**Nine millimeters is equal to 0.3543 inches.** This result is due to the conversion factor for one millimeter being 0.0394 inches. Both figures were rounded to the fourth decimal space.

The atypical conversion factor is a result of these units being from different measuring systems, millimeters from the metric system and inches from imperial system. Along with the inch, the imperial system uses the foot, yard and mile as the main forms of measurement. Part of the metric system, the millimeter is a smaller unit of measurement. Centered around the number 10, larger measuring units in the metric system include the centimeter, meter and kilometer.