PRIMARY 4 MATHEMATICS LEARNING GUIDE · BATCH 6 · GUIDE 22
Length problems become reliable when the learner separates the physical distance from the unit used to describe it. One kilometre and one thousand metres are the same length represented with different units. Compound measurements such as 3 km 250 m combine two units without changing the underlying distance.
This guide develops kilometres, metres, centimetres, compound units, conversion, addition and subtraction of lengths, multi-step distance problems, estimation and unit checks. The focus is on preserving the quantity while changing representation.
Series route: return to the Primary 4 Mathematics Learning Hub. Primary 4 measurement includes length in kilometres and conversion between compound units and smaller units; see the MOE Primary Mathematics Syllabus, updated October 2025.
1. A unit tells us the size of one measuring step
Kilometres are useful for longer distances, metres for shorter everyday distances and centimetres for smaller lengths. The number changes when the unit changes because the size of one unit changes.
The physical length itself does not change.
2. Core relationships
1 km = 1,000 m.
1 m = 100 cm.
These conversion relationships allow one measurement to be rewritten using a smaller unit.
3. Convert kilometres to metres
4 km = 4 × 1,000 = 4,000 m.
The numerical value becomes larger because metres are smaller units than kilometres.
4. Convert compound kilometres and metres to metres
3 km 250 m = 3,000 m + 250 m = 3,250 m.
Convert the larger unit first, then combine like units.
5. Convert metres to compound kilometres and metres
4,680 m contains 4 complete kilometres and 680 metres.
Therefore 4,680 m = 4 km 680 m.
Division by 1,000 identifies the number of complete kilometres and the remaining metres.
6. Convert metres and centimetres
2 m 35 cm = 200 cm + 35 cm = 235 cm.
Conversely, 347 cm = 3 m 47 cm.
The same compound-unit idea operates at a different scale.
7. Compare measurements only after making units compatible
Which is longer: 2 km 90 m or 2,150 m?
2 km 90 m = 2,090 m.
Since 2,150 m is greater, 2,150 m is longer by 60 m.
8. Add compound lengths by converting or regrouping
Add 2 km 650 m and 1 km 780 m.
Convert to metres: 2,650 + 1,780 = 4,430 m = 4 km 430 m.
Alternatively add kilometres and metres, then regroup 1,430 m as 1 km 430 m.
9. Subtract compound lengths
Find 5 km 200 m − 2 km 750 m.
Convert to metres: 5,200 − 2,750 = 2,450 m.
Answer: 2 km 450 m.
10. Regroup one kilometre when needed
5 km 200 m can also be written as 4 km 1,200 m.
Then 4 km 1,200 m − 2 km 750 m = 2 km 450 m.
Regrouping changes the representation, not the length.
11. Distance travelled in stages
A route has a first section of 1 km 850 m and a second section of 2 km 375 m.
Total distance = 1,850 m + 2,375 m = 4,225 m = 4 km 225 m.
12. Remaining distance
A trail is 8 km long. A walker has completed 5 km 650 m.
8,000 m − 5,650 m = 2,350 m.
Remaining distance = 2 km 350 m.
13. Unknown starting distance
After walking 1 km 480 m, a person has 2 km 320 m left.
Total route = 1,480 m + 2,320 m = 3,800 m = 3 km 800 m.
Distance completed and distance remaining form parts of the whole route.
14. Multiplying equal lengths
One lap is 450 m. Four laps total:
450 × 4 = 1,800 m = 1 km 800 m.
The multiplier counts equal laps.
15. Dividing a total length into equal parts
A 2,400 m route is divided into six equal sections.
2,400 ÷ 6 = 400 m per section.
The quotient is a length because the total distance is being shared.
16. Estimate before exact conversion
3 km 920 m is almost 4 km. A result claiming it equals 392 m is impossible by scale.
Estimation is especially useful when zeros are lost during conversion.
17. Choose an appropriate unit
A distance between neighbourhoods is more naturally expressed in kilometres than centimetres. A pencil length is more naturally expressed in centimetres than kilometres.
Choosing a sensible unit is part of measurement reasoning.
18. Do not add unlike units before conversion or regrouping
3 km + 250 m is not “253 km”. The numbers cannot be combined as if the units were identical.
Either keep the compound form 3 km 250 m or convert everything to one unit.
19. Common length errors
| Error | Weak link | Repair question |
|---|---|---|
| 3 km 250 m written as 3250 km | Unit not changed | What unit should the final number use? |
| 4,680 m written as 4 km 68 m | Remainder place value | How many metres remain after 4,000 m? |
| 5 km 200 m − 2 km 750 m attempted without regrouping | Compound subtraction | Would converting to metres make the relationship clearer? |
| 2 km 90 m judged larger than 2,150 m because 90>150 is ignored | Units not unified | What are both distances in metres? |
| Unit omitted | Quantity incomplete | Is the answer in km, m or cm? |
20. Practice laboratory
- Convert 6 km to metres.
- Convert 3 km 480 m to metres.
- Convert 5,720 m to km and m.
- Convert 2 m 45 cm to centimetres.
- Convert 368 cm to m and cm.
- Compare 2 km 300 m and 2,250 m.
- Add 1 km 750 m and 2 km 680 m.
- Subtract 2 km 850 m from 6 km 100 m.
- A route is 9 km long. 6 km 425 m has been completed. Find the remainder.
- One lap is 375 m. Find the distance of 6 laps.
- 2,800 m is divided into 7 equal sections. Find each section.
- A person walks 1 km 650 m and has 2 km 125 m left. Find the total route.
- Find the difference between 4 km 80 m and 3,950 m.
- Convert 7 km 5 m to metres.
- Convert 9,009 m to km and m.
- Add 850 m, 1 km 275 m and 2 km.
- A 5 m rope has 175 cm cut off. How much remains in centimetres?
- Which unit is more sensible for the distance across Singapore: km or cm?
- Explain why 2 km 500 m is not 2.500 m.
- Give an inverse check for 8,000 m − 5,650 m = 2,350 m.
21. Explained answers
1. 6,000 m.
2. 3,480 m.
3. 5 km 720 m.
4. 245 cm.
5. 3 m 68 cm.
6. 2 km 300 m = 2,300 m, so it is 50 m longer.
7. 4 km 430 m.
8. 3 km 250 m.
9. 2 km 575 m.
10. 2,250 m = 2 km 250 m.
11. 400 m.
12. 3 km 775 m.
13. 4 km 80 m = 4,080 m; difference = 130 m.
14. 7,005 m.
15. 9 km 9 m.
16. 850 + 1,275 + 2,000 = 4,125 m = 4 km 125 m.
17. 500 cm − 175 cm = 325 cm.
18. Kilometres.
19. “2 km 500 m” is a compound measurement. Writing 2.500 m means 2.5 metres, a completely different length.
20. 2,350 + 5,650 = 8,000 m.
22. Teaching routine: unit, conversion, relationship, check
First identify the unit of every given length. Convert only when needed. Preserve the meaning of the quantity through each step, and finish with a unit and scale check.
The reusable route is: name unit → unify units → calculate → regroup if useful → estimate → answer.
23. Where this guide connects next
The same compound-unit structure appears in mass and volume. Once the learner understands that a quantity can be represented in larger and smaller units without changing its physical amount, those strands become easier to organise.
Continue to Mass: Kilograms, Grams and Compound Units →
Editorial control: Wintour House V1.0 · CivDJ · eduKate Publishing.
