Converting Units of Length (Grade 4)
Length can be measured in millimetres (mm), centimetres (cm), metres (m), or kilometres (km). Converting between these units is a key skill in Class 4.
You will learn the relationship between these units and how to convert from one to another using multiplication and division.
What is Converting Units of Length - Class 4 Maths (Measurement)?
Length measures how long, wide, or tall something is. The metric units of length, from smallest to largest, are:
mm → cm → m → km
Converting Units of Length (Grade 4) Formula
1 cm = 10 mm | 1 m = 100 cm | 1 km = 1,000 m
To convert to a smaller unit → multiply.
To convert to a larger unit → divide.
| Conversion | Operation | Example |
|---|---|---|
| cm to mm | × 10 | 5 cm = 50 mm |
| mm to cm | ÷ 10 | 30 mm = 3 cm |
| m to cm | × 100 | 3 m = 300 cm |
| cm to m | ÷ 100 | 250 cm = 2 m 50 cm |
| km to m | × 1,000 | 2 km = 2,000 m |
| m to km | ÷ 1,000 | 4,500 m = 4 km 500 m |
Solved Examples
Example 1: Example 1: cm to mm
Problem: Convert 7 cm to mm.
Solution:
Step 1: 1 cm = 10 mm
Step 2: 7 cm = 7 × 10 = 70 mm
Answer: 7 cm = 70 mm.
Example 2: Example 2: mm to cm
Problem: Convert 85 mm to cm and mm.
Solution:
Step 1: 85 ÷ 10 = 8 remainder 5.
Step 2: 85 mm = 8 cm 5 mm.
Answer: 85 mm = 8 cm 5 mm.
Example 3: Example 3: m to cm
Problem: Convert 4 m 35 cm to cm.
Solution:
Step 1: 4 m = 4 × 100 = 400 cm
Step 2: 400 cm + 35 cm = 435 cm
Answer: 4 m 35 cm = 435 cm.
Example 4: Example 4: cm to m
Problem: Convert 670 cm to m and cm.
Solution:
Step 1: 670 ÷ 100 = 6 remainder 70.
Step 2: 670 cm = 6 m 70 cm.
Answer: 670 cm = 6 m 70 cm.
Example 5: Example 5: km to m
Problem: Convert 3 km 200 m to metres.
Solution:
Step 1: 3 km = 3 × 1,000 = 3,000 m
Step 2: 3,000 m + 200 m = 3,200 m
Answer: 3 km 200 m = 3,200 m.
Example 6: Example 6: m to km
Problem: Aman's school is 2,750 m from home. Express this in km and m.
Solution:
Step 1: 2,750 ÷ 1,000 = 2 remainder 750.
Step 2: 2,750 m = 2 km 750 m.
Answer: The school is 2 km 750 m away.
Example 7: Example 7: Adding lengths with different units
Problem: Add 3 m 45 cm and 2 m 70 cm.
Solution:
Step 1: Add cm: 45 + 70 = 115 cm = 1 m 15 cm.
Step 2: Add m: 3 + 2 + 1 = 6 m.
Step 3: Total = 6 m 15 cm.
Answer: 6 m 15 cm.
Example 8: Example 8: Subtracting lengths
Problem: Priya has a ribbon 5 m 20 cm long. She cuts off 2 m 60 cm. How much is left?
Solution:
Step 1: Convert: 5 m 20 cm = 520 cm. 2 m 60 cm = 260 cm.
Step 2: 520 − 260 = 260 cm = 2 m 60 cm.
Answer: 2 m 60 cm of ribbon is left.
Example 9: Example 9: Comparing lengths
Problem: Which is longer: 3 km 400 m or 3,500 m?
Solution:
Step 1: Convert 3 km 400 m to m: 3,000 + 400 = 3,400 m.
Step 2: Compare: 3,400 m vs 3,500 m.
Step 3: 3,500 > 3,400.
Answer: 3,500 m is longer.
Example 10: Example 10: Word problem
Problem: Kavi walks 1 km 800 m to school in the morning and 1 km 800 m back in the evening. How far does he walk each day?
Solution:
Step 1: Convert: 1 km 800 m = 1,800 m.
Step 2: Total = 1,800 + 1,800 = 3,600 m = 3 km 600 m.
Answer: Kavi walks 3 km 600 m each day.
Key Points to Remember
- 1 cm = 10 mm, 1 m = 100 cm, 1 km = 1,000 m.
- To convert to a smaller unit, multiply.
- To convert to a larger unit, divide.
- To add or subtract lengths in different units, first convert to the same unit.
- Choose the appropriate unit: mm for pencil thickness, cm for notebook length, m for room length, km for city distances.
Practice Problems
- Convert 9 cm to mm.
- Convert 4,200 m to km and m.
- Add: 5 m 80 cm + 3 m 45 cm.
- Subtract: 7 km 300 m − 4 km 750 m.
- Which is greater: 450 cm or 5 m?
- A rope is 12 m 50 cm long. It is cut into 5 equal pieces. How long is each piece?
- Neha's height is 1 m 35 cm. Express it in cm only.
Frequently Asked Questions
Q1. How many mm are in 1 cm?
There are 10 mm in 1 cm.
Q2. How many cm are in 1 m?
There are 100 cm in 1 m.
Q3. How many m are in 1 km?
There are 1,000 m in 1 km.
Q4. How do you convert a bigger unit to a smaller unit?
Multiply. For example, 3 m to cm: 3 × 100 = 300 cm. You get more of the smaller unit.
Q5. How do you convert a smaller unit to a bigger unit?
Divide. For example, 500 cm to m: 500 ÷ 100 = 5 m. You get fewer of the larger unit.
Q6. How do you add lengths with different units?
Convert both lengths to the same unit first, then add. For example, 2 m 30 cm + 1 m 80 cm: convert to cm (230 + 180 = 410 cm = 4 m 10 cm).
Q7. When should I use mm, cm, m, or km?
Use mm for very small things (thickness of a coin), cm for everyday objects (pencil, notebook), m for rooms and buildings, and km for distances between towns.
Q8. What is the full form of km, m, cm, and mm?
km = kilometre, m = metre, cm = centimetre, mm = millimetre. The prefix kilo means 1,000, centi means 1/100, and milli means 1/1,000.
Related Topics
- Converting cm to m and m to cm
- Converting Units of Weight (Grade 4)
- Perimeter (Grade 4)
- Introduction to Area
- Area of Rectangle (Grade 4)
- Area of Square (Grade 4)
- Area and Perimeter Word Problems (Grade 4)
- Converting Units of Capacity (Grade 4)
- Area by Counting Squares
- Perimeter of Irregular Shapes
- Relationship Between Area and Perimeter
