Length — Grade 3 Mathematics
In Grade 3, we work confidently with all four metric length units: mm, cm, m and km. We convert between them, compare measurements, and apply length to perimeter problems with mixed units.
Length Units and Conversions
mm
÷ 10→× 10
cm
÷ 100→× 100
m
÷ 1000→× 1000
km
| Relationship | To convert smaller → larger | To convert larger → smaller |
|---|---|---|
| 10 mm = 1 cm | mm ÷ 10 = cm | cm × 10 = mm |
| 100 cm = 1 m | cm ÷ 100 = m | m × 100 = cm |
| 1 000 m = 1 km | m ÷ 1000 = km | km × 1000 = m |
💡 Rule: Going to a bigger unit → divide. Going to a smaller unit → multiply.
Worked Examples
Example 1 — Conversions
Convert: (a) 3.5 km to m (b) 450 cm to m (c) 28 mm to cm
- (a) 3.5 × 1 000 = 3 500 m
- (b) 450 ÷ 100 = 4.5 m
- (c) 28 ÷ 10 = 2.8 cm
Example 2 — Comparing Mixed Units
Which is longer: 2.4 m or 250 cm?
- Convert to the same unit: 2.4 m = 2.4 × 100 = 240 cm
- 240 cm vs 250 cm → 250 cm is longer
- 250 cm (= 2.5 m) is longer.
Example 3 — Real World: School Run 🏃
Layla walks 750 m to school, then 1.2 km to her friend's house. How far in total? Give your answer in km.
- Convert 750 m to km: 750 ÷ 1 000 = 0.75 km
- Total = 0.75 + 1.2 = 1.95 km
Practice Questions
1. Convert 6.2 km to metres.
✅ 6.2 × 1 000 = 6 200 m
2. Convert 350 mm to cm.
✅ 350 ÷ 10 = 35 cm
3. Order from shortest to longest: 1.5 m, 145 cm, 160 cm, 1.4 m.
✅ Convert to cm: 150, 145, 160, 140 → 1.4 m, 145 cm, 1.5 m, 160 cm
4. A rectangle is 3.5 m long and 1.8 m wide. What is its perimeter in metres?
✅ P = 2(3.5 + 1.8) = 2 × 5.3 = 10.6 m
5. A road is 4 500 m. Write this in km.
✅ 4 500 ÷ 1 000 = 4.5 km
Key Points to Remember
- 10 mm = 1 cm; 100 cm = 1 m; 1 000 m = 1 km.
- Smaller → larger unit: divide. Larger → smaller unit: multiply.
- To compare measurements: convert to the same unit first.
- Decimals in length: 1.5 m = 1 m 50 cm; 2.75 km = 2 750 m.
- Perimeter of shapes uses length — add all sides in the same unit.