CC.2 Convert and compare metric units of length
What is the metric system?
The metric system is a decimal system of measurement used all over the world. It is based on powers of ten, which makes converting between units simple. The base unit for length in the metric system is the meter.
- A meter is about the height of a doorknob.
- A kilometer is used to measure long distances, like between cities.
- A centimeter is used to measure small objects, like a pencil.
The metric system is also called the International System of Units, or SI. It is used in science, medicine, and in most countries around the world.
Metric units of length
There are several metric units for measuring length. The most common ones are kilometer (km), meter (m), centimeter (cm), and millimeter (mm). Each unit is ten times larger or smaller than the next.
- 1 kilometer = 1,000 meters
- 1 meter = 100 centimeters
- 1 centimeter = 10 millimeters
The prefixes kilo-, centi-, and milli- tell you how many times to multiply or divide by ten. For example, kilo- means 1,000.
Converting metric units of length
Converting means changing a measurement from one unit to another without changing the actual length. To convert, you multiply or divide by powers of ten. When you go from a larger unit to a smaller unit, you multiply. When you go from a smaller unit to a larger unit, you divide.
- 2 meters = 200 centimeters (2 × 100 = 200)
- 3,000 meters = 3 kilometers (3,000 ÷ 1,000 = 3)
- 5 centimeters = 50 millimeters (5 × 10 = 50)
Use a place value chart or a conversion diagram to help you remember whether to multiply or divide.
Using multiplication to convert
When you convert from a larger unit to a smaller unit, you multiply by the number of smaller units in one larger unit. This is because it takes more of the smaller unit to make the same length.
- 4 kilometers = 4,000 meters (4 × 1,000 = 4,000)
- 7 meters = 700 centimeters (7 × 100 = 700)
- 9 centimeters = 90 millimeters (9 × 10 = 90)
Always check your answer by asking: Does the number get larger when I use a smaller unit? Yes, it should.
Using division to convert
When you convert from a smaller unit to a larger unit, you divide by the number of smaller units in one larger unit. This is because it takes fewer of the larger unit to make the same length.
- 5,000 meters = 5 kilometers (5,000 ÷ 1,000 = 5)
- 800 centimeters = 8 meters (800 ÷ 100 = 8)
- 60 millimeters = 6 centimeters (60 ÷ 10 = 6)
Always check your answer by asking: Does the number get smaller when I use a larger unit? Yes, it should.
Comparing metric units of length
Comparing means deciding which length is longer, shorter, or if they are equal. To compare measurements, they must be in the same unit. If they are not, convert one or both measurements first.
- 1 meter > 50 centimeters (100 cm > 50 cm)
- 2 kilometers = 2,000 meters
- 3,000 millimeters < 4 meters (3,000 mm < 4,000 mm)
Use the symbols > for greater than, < for less than, and = for equal to.
Solving word problems with metric units
Word problems often ask you to convert and compare metric units. Read carefully to find the units used. Then convert to the same unit before you compare or calculate.
- Maria ran 2 kilometers. Leo ran 1,500 meters. Who ran farther?
2 km = 2,000 m, so Maria ran farther. - A rope is 3 meters long. Another rope is 250 centimeters long. Which is longer?
3 m = 300 cm, so the first rope is longer. - A pencil is 18 centimeters. A crayon is 180 millimeters. Are they equal?
18 cm = 180 mm, so yes, they are equal.
Always label your answer with the correct unit. This shows you understand the measurement.
Metric conversion chart
A conversion chart helps you remember how units relate to each other. You can use it to convert quickly and accurately.
- 1 kilometer (km) = 1,000 meters (m)
- 1 meter (m) = 100 centimeters (cm)
- 1 centimeter (cm) = 10 millimeters (mm)
- 1 meter (m) = 1,000 millimeters (mm)
You can also use this chart to convert between units that are not directly next to each other. For example, 1 kilometer = 100,000 centimeters.
Common mistakes to avoid
Students often make mistakes when converting or comparing metric units. Here are some common errors and how to avoid them.
- Mistake: Multiplying when you should divide.
Fix: Remember, smaller to larger means divide. - Mistake: Forgetting to convert before comparing.
Fix: Always convert to the same unit first. - Mistake: Misplacing the decimal point.
Fix: Use a place value chart to check your work.
Practice regularly to build confidence and accuracy in converting and comparing metric units.
Real-world applications
Understanding metric units of length is important in many real-world situations, such as science experiments, sports, and travel.
- A scientist measures the length of a leaf in centimeters.
- A runner tracks a 5-kilometer race.
- A traveler checks the distance between two cities in kilometers.
Being able to convert and compare metric units helps you understand information in charts, maps, and scientific reports.
Common Core alignment: CCSS.MATH.CONTENT.4.MD.A.1 – Know relative sizes of measurement units within one system of units including km, m, cm; mm. Within a single system of measurement, express measurements in a larger unit in terms of a smaller unit.
Notes for teachers
This resource is designed to provide clear explanations, examples, and practice for learners of different needs and levels of support. It may be used for whole-class instruction, guided practice, independent learning, review, homework, or additional reinforcement at home.