Weight & Mass — Grade 5 Mathematics
In Grade 5, we work confidently with metric mass, recognise approximate imperial equivalents, and solve complex multi-step problems involving mass in real-world contexts including recipes and packaging.
Conversions: Metric and Imperial
| Conversion | Rule | Example |
|---|---|---|
| g ↔ kg | ÷1 000 / ×1 000 | 3 450 g = 3.45 kg |
| kg ↔ t (tonne) | ÷1 000 / ×1 000 | 2.5 t = 2 500 kg |
| lb → kg (approx) | ÷ 2.2 (1 lb ≈ 450 g) | 11 lb ≈ 5 kg |
| kg → lb (approx) | × 2.2 (1 kg ≈ 2.2 lb) | 8 kg ≈ 17.6 lb |
💡 Tonne: 1 tonne (t) = 1 000 kg. Used for very heavy objects such as trucks and whales.
Worked Examples
Example 1 — g, kg, t
A lorry carries 3.75 t of goods. Express this in kg and g.
- 3.75 t × 1 000 = 3 750 kg
- 3 750 kg × 1 000 = 3 750 000 g
Example 2 — Recipe Scaling
A recipe for 4 people uses 1.2 kg of chicken. What mass is needed for 10 people?
- For 1 person: 1.2 ÷ 4 = 0.3 kg
- For 10 people: 0.3 × 10 = 3 kg
Example 3 — Imperial Conversion ✈️
A suitcase weighs 22 lb. The airline limit is 23 kg. Is it within the limit?
- 22 lb ÷ 2.2 = 10 kg
- 10 kg < 23 kg — Yes, it is within the limit ✓
Practice Questions
1. Convert 4.85 t to kg.
✅ 4.85 × 1 000 = 4 850 kg
2. Convert 8 800 g to kg.
✅ 8 800 ÷ 1 000 = 8.8 kg
3. Convert 55 lb to kg (use 1 kg ≈ 2.2 lb).
✅ 55 ÷ 2.2 = 25 kg
4. A parcel can hold a maximum of 2.5 kg. It already has 875 g. How much more can be added in grams?
✅ 2.5 kg = 2 500 g; 2 500 − 875 = 1 625 g
5. 24 identical tins total 7.2 kg. What is the mass of one tin in grams?
✅ 7.2 ÷ 24 = 0.3 kg = 300 g
Key Points to Remember
- 1 000 g = 1 kg; 1 000 kg = 1 tonne (t).
- 1 kg ≈ 2.2 lb (pounds); 1 lb ≈ 450 g.
- Convert to same unit before adding, subtracting, or comparing.
- Multi-step: find 1 unit first, then scale up or down.
- Tonnes are used for very large masses (vehicles, building materials).