Centimeters and millimeters are both fractional amounts of a metric meter. One centimeter is 1/100 of a meter and 1 millimeter is 1/1,000 of a meter. Therefore, each centimeter is equal to 10 millimeters.
Calculate length in centimeters
Measure the length of the object in centimeters using a standard ruler or measuring tape. Record the answer.
Multiply the number of centimeters by 10
Multiply the number of centimeters by 10. For example, if a board measures 42 centimeters, take 42 and multiply it by 10. The board measures 420 millimeters.
Record the answer in millimeters
Record the converted amount, such as 420 millimeters.