What is a Millimeter? - Definition & Conversion

Instructor: T.J. Hoogsteen

T.J. is currently a grade 5 teacher and Vice-Principal. He has a master's degree in Educational Administration and is working toward an Ed.D. in Educational Leadership.

Have you ever looked at a ruler and wondered what the little lines were, or saw 'mm' and didn't know what it meant? 'MM' stands for millimeter. In this lesson, you will learn about what a millimeter is, and what it compares to in metric units, as well as imperial measurements.

What is a mm?

Imagine you are helping out a friend as they design a poster to advertise for the upcoming school fundraiser. Your friend asks you to measure a few lines and then hands you this ruler:


What is this? You haven't seen a ruler like this before. What is a 'mm'? If this is something that you are wondering, you're in luck, because that is exactly what this lesson will cover.

So what is a mm? First of all, mm is the abbreviation for millimeter. A millimeter is a metric unit used to measure the length of an object. Sometimes, millimeter will be spelled as 'millimetre'. Both ways are correct and both mean the same thing.

Millimeters are used to measure small objects. Going back to the picture of the ruler, you may notice the longer lines with numbers underneath them. These numbers are actually marking centimeters. The smaller lines indicate millimeters. There are 10 millimeters in 1 centimeter. There are 1000 millimeters in 1 meter (another metric unit of length for measuring larger objects, comparable to a yard).

Many people are still familiar with imperial, or standard, units of measure like inches, feet, and yards. Millimeters can be compared to each of these units of measurement. To show how small a millimeter really is, 1 millimeter is equal to 0.03937 inches, or 5/127 of an inch. There are 25.4 millimeters in 1 inch. In 1 foot, there are 304.8 millimeters, and in 1 yard there are 914.4 millimeters. There's not really a fancy way to remember the conversions, so memorizing them with the help of flash cards could be useful.

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