PRIMARY 4 MATHEMATICS LEARNING GUIDE · BATCH 6 · GUIDE 24
Liquid-volume problems become manageable when litres and millilitres are treated as two representations of the same quantity. A container holding 2 litres 350 millilitres holds exactly the same amount as one labelled 2,350 millilitres.
This guide develops litres, millilitres, compound units, conversion, addition and subtraction, pouring, remaining volume, equal sharing, estimation and multi-step word problems. The learner’s job is to preserve the quantity while the unit representation changes.
Series route: return to the Primary 4 Mathematics Learning Hub. Primary 4 includes measuring liquid volume in millilitres and working with compound measurement units; see the MOE Primary Mathematics Syllabus, updated October 2025.
1. Litres and millilitres measure liquid volume
Litres are useful for larger liquid quantities. Millilitres describe smaller amounts more conveniently.
The amount of liquid does not change when we change its unit.
2. Core relationship
1 L = 1,000 mL.
This relationship connects compound forms such as 3 L 250 mL to a single smaller unit.
3. Convert litres to millilitres
4 L = 4 × 1,000 = 4,000 mL.
The number becomes larger because millilitres are smaller units.
4. Convert compound litres and millilitres to millilitres
2 L 375 mL = 2,000 mL + 375 mL = 2,375 mL.
Convert the litre part first, then combine like units.
5. Convert millilitres to litres and millilitres
4,860 mL contains four complete litres and 860 mL.
Therefore 4,860 mL = 4 L 860 mL.
6. Compare volumes only after the units agree
Which is greater: 2 L 90 mL or 2,150 mL?
2 L 90 mL = 2,090 mL.
Therefore 2,150 mL is greater by 60 mL.
7. Add liquid volumes
Add 1 L 650 mL and 2 L 780 mL.
1,650 mL + 2,780 mL = 4,430 mL.
Answer: 4 L 430 mL.
8. Add by regrouping millilitres into litres
650 mL + 780 mL = 1,430 mL = 1 L 430 mL.
Adding the litre parts then gives 4 L 430 mL.
The regrouping is exact because 1,000 mL is one litre.
9. Subtract compound volumes
Find 5 L 200 mL − 2 L 750 mL.
Convert to millilitres: 5,200 − 2,750 = 2,450 mL.
Answer: 2 L 450 mL.
10. Regroup one litre when subtracting
5 L 200 mL can be rewritten as 4 L 1,200 mL.
Then 4 L 1,200 mL − 2 L 750 mL = 2 L 450 mL.
The physical amount has not changed; only the representation has.
11. Volume remaining after pouring
A jug contains 6 L of water. 2 L 375 mL is poured out.
6,000 − 2,375 = 3,625 mL.
Remaining volume = 3 L 625 mL.
12. Reconstruct an original volume
After 1 L 480 mL is poured out, 2 L 320 mL remains.
Original amount = 1,480 + 2,320 = 3,800 mL = 3 L 800 mL.
The poured and remaining parts reconstruct the original whole.
13. Several pouring events need intermediate states
A container begins with 5 L. First 850 mL is poured out, then 1 L 275 mL is added.
After removal: 5,000 − 850 = 4,150 mL.
After addition: 4,150 + 1,275 = 5,425 mL = 5 L 425 mL.
14. Equal bottles use multiplication
One bottle contains 450 mL. Four bottles contain:
450 × 4 = 1,800 mL = 1 L 800 mL.
The multiplier counts equal bottles.
15. Equal sharing uses division
2,400 mL is poured equally into six cups.
2,400 ÷ 6 = 400 mL per cup.
The quotient is a liquid volume per container.
16. Capacity and actual volume are not the same statement
A bottle with capacity 1 L can hold up to 1,000 mL. If it currently contains 650 mL, its actual liquid volume is 650 mL, not 1 L.
Capacity describes the maximum amount a container can hold; the current volume describes what is inside now.
17. Find unused capacity
A 2 L container currently holds 1 L 350 mL.
Unused capacity = 2,000 − 1,350 = 650 mL.
This is a missing-part problem: current liquid plus unused capacity equals total capacity.
18. Estimate before exact calculation
3 L 920 mL is almost 4 L. A conversion result of 392 mL is impossible by scale.
Adding about 2 L and 3 L should produce roughly 5 L, not 50 L.
19. Choose a sensible unit
A medicine spoon or small drink sample may be measured in millilitres. A large jug or bucket may be more naturally described in litres.
Appropriate units make measurements easier to interpret and compare.
20. Do not combine litres and millilitres as bare numbers
3 L + 250 mL is not 253 L.
Write 3 L 250 mL or convert the whole quantity to 3,250 mL.
21. Common liquid-volume errors
| Error | Weak link | Repair question |
|---|---|---|
| 2 L 80 mL written as 2,800 mL | Compound-unit place value | How many mL are in 2 L before adding 80? |
| 4,860 mL written as 4 L 86 mL | Remainder misread | What remains after 4,000 mL? |
| Capacity treated as current amount | Two quantities confused | Is the question asking what can fit or what is present? |
| Volume removed but final amount grows | Direction check missing | Did liquid leave or enter? |
| Answer has no unit | Quantity incomplete | Litres or millilitres? |
22. Practice laboratory
- Convert 7 L to mL.
- Convert 3 L 450 mL to mL.
- Convert 5,208 mL to L and mL.
- Compare 2 L 75 mL and 2,120 mL.
- Add 1 L 825 mL and 2 L 650 mL.
- Subtract 2 L 780 mL from 6 L 100 mL.
- A container has 8 L. 3 L 425 mL is poured out. Find the remainder.
- One bottle contains 375 mL. Find the total in six bottles.
- 2,800 mL is shared equally among seven cups. Find each share.
- After 1 L 650 mL is poured out, 2 L 125 mL remains. Find the original amount.
- Find the difference between 4 L 80 mL and 3,950 mL.
- Convert 9 L 5 mL to mL.
- Convert 10,009 mL to L and mL.
- Add 850 mL, 1 L 275 mL and 2 L.
- A 5 L container has 1,750 mL removed. Find the remainder in mL.
- A 2 L bottle contains 1 L 350 mL. Find unused capacity.
- A 750 mL bottle contains 500 mL. Is its capacity 500 mL or 750 mL?
- Estimate the total of 2 L 950 mL and 3 L 120 mL.
- Explain why 2 L 500 mL is not 2.500 mL.
- Give an inverse check for 6,000 mL − 2,375 mL = 3,625 mL.
23. Explained answers
1. 7,000 mL.
2. 3,450 mL.
3. 5 L 208 mL.
4. 2 L 75 mL = 2,075 mL, so 2,120 mL is greater by 45 mL.
5. 4 L 475 mL.
6. 3 L 320 mL.
7. 4 L 575 mL.
8. 2,250 mL = 2 L 250 mL.
9. 400 mL.
10. 3 L 775 mL.
11. 4,080 − 3,950 = 130 mL.
12. 9,005 mL.
13. 10 L 9 mL.
14. 4,125 mL = 4 L 125 mL.
15. 5,000 − 1,750 = 3,250 mL.
16. 650 mL.
17. Capacity is 750 mL; current volume is 500 mL.
18. About 6 L.
19. “2 L 500 mL” is a compound volume equal to 2,500 mL. “2.500 mL” would mean 2.5 millilitres.
20. 3,625 + 2,375 = 6,000 mL.
24. Teaching routine: preserve the amount of liquid
Ask whether each number describes capacity, current volume, amount poured, amount remaining or an equal share. Convert to a common unit only when it clarifies the calculation, then regroup and check the scale.
The reusable route is: name quantity → name unit → convert → calculate → regroup → estimate → verify.
25. Batch 6 world return
This batch fills four practical measurement-and-money gaps beneath the Primary 4 hub:
- Money in Decimal Notation, Change, Budgeting and Word Problems
- Length: Kilometres, Metres, Centimetres and Compound Units
- Mass: Kilograms, Grams and Compound Units
- Volume of Liquid: Litres, Millilitres and Compound Units — this guide.
The common learning principle is unit preservation: the representation can change while the physical or monetary quantity remains controlled.
Editorial control: Wintour House V1.0 · CivDJ · eduKate Publishing.
