Lesson 69: Millimeters

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.

A millimeter (mm) is one-tenth of a centimeter ($1$ cm $= 10$ mm) and one-thousandth of a meter ($1$ m $= 1000$ mm). A dime's thickness is a great real-world example of 1 millimeter.

A segment that is 5 cm long is equivalent to $5 \times 10 = 50$ mm long.
A stack of 10 dimes would be approximately 10 mm tall, which is equal to 1 cm.
The thickness of a standard paper clip wire is about 1 mm.

Think of a centimeter as a small hop and a millimeter as a tiny step. You need to take ten tiny millimeter steps to cover the same distance as one centimeter hop. This super small unit is perfect for measuring tiny things accurately, like the thickness of your fingernail or a credit card. It’s the ultimate tool for precision!

The abbreviation for centimeter is cm. The abbreviation for millimeter is mm.

A length of 4 centimeters is properly written as 4 cm.
A measurement of 45 millimeters is correctly abbreviated as 45 mm.
To show a conversion, you would write: $3$ cm $= 30$ mm.

Why write out long words when you can use a secret code? Using 'cm' for centimeters and 'mm' for millimeters is the official math shortcut. It keeps your notes clean, saves you time, and makes your work look professional. It’s like texting 'LOL' instead of 'laughing out loud'—but for measurement! Be efficient, be cool, use the symbols.

To convert from centimeters to millimeters, you multiply the number of centimeters by 10. To convert from millimeters to centimeters, you divide by 10, which can be done by moving the decimal point one place to the left.

A pencil measuring 18 cm long is $18 \times 10 = 180$ mm long.
A bug that is 25 mm long can also be described as $25 \div 10 = 2.5$ cm long.
A house key that is 5.2 cm long is equivalent to $5.2 \times 10 = 52$ mm long.

Jumping between millimeters and centimeters is super easy! Since every centimeter holds ten millimeters, you just multiply by 10 to go from cm to mm. Need to go the other way? Just divide by 10. This is awesome for turning millimeters into a decimal centimeter format, like changing 47 mm into a neat 4.7 cm.

To find the length of a part of a line segment, subtract the length of the known part from the total length of the segment. When measurements are in centimeters with millimeters, use decimal subtraction for the calculation.

A total segment $AC$ is 8.5 cm long. If part $AB$ is 3.2 cm, then segment $BC$ is $8.5 - 3.2 = 5.3$ cm.
A segment $XZ$ is 15.9 cm long. If part $YZ$ is 4.7 cm, the length of $XY$ is $15.9 - 4.7 = 11.2$ cm.

Imagine a licorice rope is 12.5 cm long, and your friend bites off a 3.1 cm piece. To find out how much licorice you have left, you just subtract! Using decimals makes it simple to work with these precise centimeter-and-millimeter measurements. It's just like regular subtraction, but with a decimal point to keep things perfectly aligned and accurate.