How to Read mm on a Ruler

By Tamara Christine Van Hooser; Updated April 25, 2017

If all the fractions involved in reading a standard English ruler confuse you, it may relieve you to know that the fractions disappear when reading millimeters on a metric ruler. One millimeter equals one-tenth of a centimeter and you express any measurement that falls between whole numbers as "point n" where "n" equals the number of tiny, intermediary hash marks on the ruler. Since you are dealing with tenths, the fractional equivalent would always be "n/10," so you can skip the fraction and jump right to the decimal format.

Examine the metric ruler. The longer numbered hash marks are the centimeters. Count the hash marks between two whole numbers to see that the centimeters divide into ten increments. Each of these increments is a millimeter (mm).

Lay the ruler against a small object such as a crayon or a playing card, lining up the end with the edge of the object. Count how many whole numbers, four, for example. This indicates the centimeters. Since you know that there are 10 millimeters (mm) in every centimeter, you can multiply the centimeters by 10 to calculate that for centimeters equals 40 millimeters.

Count the intermediary hash marks past the last whole number, for example seven. Each hash mark represents one millimeter, so you can read that as seven millimeters or 0.7 centimeters.

Combine the two and you get a measurement of four centimeters seven millimeters, 4.7 centimeters or 47 millimeters.

About the Author

Tamara Christine has written more than 900 articles for a variety of clients since 2010. She holds a Bachelor of Arts in applied linguistics and an elementary teaching license. Additionally, she completed a course in digital journalism in 2014. She has more than 10 years experience teaching and gardening.