Millimeters
Millimeters are introduced as a unit of measurement in Grade 4, Saxon Math Intermediate 4 Chapter 7. One centimeter equals 10 millimeters, making the millimeter a very small unit suitable for measuring things like coin thickness or hair strands. Converting centimeters to millimeters requires multiplying by 10 because you are going from a larger unit to a smaller unit. For example, a 9 cm crayon equals 9 times 10 equals 90 mm. A common error is dividing instead of multiplying—remember that more millimeters are needed to express the same length.
Key Concepts
New Concept If we divide a centimeter into ten equal lengths, each equal length is 1 millimeter long. A millimeter is one thousandth ($\frac{1}{1000}$) of a meter.
What’s next Next, you'll practice converting between millimeters and centimeters and use these new units to measure and compare objects around you.
Common Questions
How many millimeters are in one centimeter?
There are 10 millimeters in 1 centimeter: 1 cm equals 10 mm.
How do I convert centimeters to millimeters?
Multiply the number of centimeters by 10. For example, 9 cm times 10 equals 90 mm.
How do I convert millimeters to centimeters?
Divide the number of millimeters by 10. For example, 40 mm divided by 10 equals 4 cm.
What objects are typically measured in millimeters?
Very small objects like coin thickness, pencil lead width, or fine wire diameter are measured in millimeters.
Why do I multiply (not divide) when converting cm to mm?
A millimeter is smaller than a centimeter, so you need more of them to express the same length. Going from a larger unit to a smaller unit always requires multiplying.