Converting Millimeters and Centimeters
Grade 4 students learn to convert between millimeters and centimeters in Saxon Math Intermediate 4 Chapter 7 using the simple rule: 10 millimeters = 1 centimeter. To convert centimeters to millimeters (larger to smaller unit), multiply by 10—a 5.2 cm key is 52 mm. To convert millimeters to centimeters (smaller to larger unit), divide by 10—a 32 mm paperclip is 3.2 cm. Proper abbreviations (mm, cm, m) are never pluralized; 10 mm is always written as 10 mm, never 10 mms.
Key Concepts
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.
Common Questions
How many millimeters are in one centimeter?
There are exactly 10 millimeters (mm) in one centimeter (cm). This 10-to-1 relationship is the key conversion factor between these two metric units of length.
How do you convert centimeters to millimeters?
Multiply the number of centimeters by 10. For example, 8.5 cm × 10 = 85 mm. You need more millimeters than centimeters because mm is the smaller unit.
How do you convert millimeters to centimeters?
Divide the number of millimeters by 10. For example, 32 mm ÷ 10 = 3.2 cm. You get fewer centimeters because cm is the larger unit, so one cm contains 10 mm.
How do you remember whether to multiply or divide when converting?
If you convert to a smaller unit (cm to mm), multiply—you need more of the smaller units. If you convert to a larger unit (mm to cm), divide—you need fewer of the larger units.
What are the correct abbreviations for metric units of length?
Millimeter is mm, centimeter is cm, and meter is m. These abbreviations are symbols and are never pluralized—write 10 mm, not 10 mms. The abbreviation stays the same regardless of the quantity.
What everyday objects are best measured in millimeters versus centimeters?
Millimeters are used for very small objects like pencil lead diameter (0.7 mm), pill thickness, or fine engravings. Centimeters are used for slightly larger items like book width, screen size, or a hand width.