What is a Millimeter? – Definition & Conversion

Christianlly has taught college Physics, Natural science, Earth science, and facilitated laboratory courses. He has a master’s degree in Physics and is currently pursuing his doctorate degree. View bio

Learn about the millimeter, a small unit of length measurement in the metric system. Discover how many millimeters are in a centimeter and a meter, and how to convert millimeters to the units in the imperial measurement system. Updated: 12/22/2023

What Is a Millimeter?

Take a second to imagine you’re 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.
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Imperial vs Metric

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,

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Examples

Millimeters are used to measure in many applications around the world. They’re used especially when the accuracy for measurement needs to be more precise than inches. A common use for millimeters is in cutting hair. A pair of hair clippers set to Level 1, will cut hair to 3 millimeters long.

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Lesson Summary

Okay, let’s take a moment or two to review. As we learned in this lesson, millimeters are a small metric unit for measuring length. They’re useful when measuring small objects, or when measurements need to be more precise than centimeters.

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Additional Activities

Millimeter: Word Problem

In this activity, you will check your knowledge of how to convert millimeters to other metric units.

Directions

For this activity, print or copy this page on a blank piece of paper. Then, carefully read the given word problems and provide a solution.

1.) Ken needs to buy two yards of red fabric to make a costume for a school play. The store only has six feet of red fabric left. Will it be enough for Ken to make his costume?

2.) How many bracelets can be made from three meters of string if each bracelet needs thirty millimeters of string?

3.) You need a table that has a height of at least sixty centimeters. You find one at a shop with six hundred twenty-five millimeters. Convert the unit from centimeters to millimeters if this table will suffice your needs.

4.) A stick that is thirty inches long has to be cut into six smaller pieces. How many millimeters will each smaller piece of the stick measure?

Answer Key

1.) 1 yard = 914.4 millimeters, 1 foot = 304.8 millimeters

914.4 / 304. 8 = 3 feet

This means that 1 yard = 3 feet

Yes. Ken will be able to make his costume with 6 feet of fabric.

2.) 1 meter = 1000 millimeters

1000 x 3 = 3000 millimeters

100 bracelets can be made.

3.) 1 centimeter = 10 millimeters

The table has a height of 62. 5 centimeters and fits my needs.

4.) 1 inch = 25.4 millimeters

30 x 25.4 = 762 millimeters

762 / 6 = 127 millimeters

Each stick will be 127 millimeters long.