Length — Grade 4 Mathematics
In Grade 4, we write lengths using decimal notation (e.g., 3.45 m), convert fluently between all metric units, and solve multi-step length problems involving addition, subtraction, and multiplication.
Metric Length Conversions
mm
÷ 10⟺× 10
cm
÷ 100⟺× 100
m
÷ 1000⟺× 1000
km
Decimal notation:
• 5 m 37 cm = 5.37 m • 2 km 450 m = 2.45 km • 8 cm 6 mm = 8.6 cm
Quick reference:
• 1 km = 1 000 m • 1 m = 100 cm • 1 cm = 10 mm
• 5 m 37 cm = 5.37 m • 2 km 450 m = 2.45 km • 8 cm 6 mm = 8.6 cm
Quick reference:
• 1 km = 1 000 m • 1 m = 100 cm • 1 cm = 10 mm
💡 Tip: To convert a smaller unit to a larger unit, divide. To convert a larger unit to a smaller unit, multiply.
Worked Examples
Example 1 — Convert and Express as Decimal
Write 4 km 75 m as km using decimals.
- 75 m out of 1 000 m = 75 ÷ 1 000 = 0.075 km
- 4 km + 0.075 km = 4.075 km
Example 2 — Add Mixed Units
Add 2.35 m + 87 cm. Give your answer in metres.
- Convert 87 cm → 0.87 m
- 2.35 + 0.87 = 3.22 m
Example 3 — Multi-Step Problem 🚴
A cyclist rides 3.6 km in the morning and 2 750 m in the afternoon. How far in total? Give the answer in km.
- Convert 2 750 m → 2.75 km
- 3.6 + 2.75 = 6.35 km
Practice Questions
1. Convert 8 cm 4 mm to mm.
✅ 8 × 10 + 4 = 84 mm
2. Write 6 m 25 cm in metres using decimals.
✅ 6.25 m
3. Convert 3.8 km to metres.
✅ 3.8 × 1 000 = 3 800 m
4. A fence has 12 panels, each 1.75 m long. What is the total length of the fence?
✅ 12 × 1.75 = 21 m
5. A rope is 5.4 m long. A piece of 85 cm is cut off. How much is left? Give your answer in metres.
✅ 85 cm = 0.85 m; 5.4 − 0.85 = 4.55 m
Key Points to Remember
- 1 km = 1 000 m; 1 m = 100 cm; 1 cm = 10 mm.
- Decimal notation: 3 m 45 cm = 3.45 m.
- Smaller → larger unit: divide by the conversion factor.
- Larger → smaller unit: multiply by the conversion factor.
- Always convert to the same unit before adding or subtracting.