Measurements — Class 4 Mathematics (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 4 Mathematics, Unit 4. Relationship between metre and centimetre, conversion, addition and subtraction of standard measurements, and estimation. Taught in August.
1. Why standard units
In Class 3 you measured with handspans and footspans. The trouble is that your handspan and your teacher's handspan are different lengths, so the same table gives two different answers.
Standard units fix that. A centimetre is a centimetre in every classroom in Tamil Nadu.
2. The units and how they relate
| Relationship | Meaning |
|---|---|
| 10 millimetre = 1 centimetre | The smallest marks on your ruler |
| 100 centimetre = 1 metre | A metre scale carries 100 cm |
| 1000 metre = 1 kilometre | Distances between towns |
The textbook also notes that 1 mile is about 1 kilometre 600 metre.
Which unit for which job? Measure the tip of a pencil in millimetres, your toy car in centimetres (Sharmila's measures 7 cm, Banu's toy lorry 25 cm), the classroom door in metres, and the road from Villupuram to Cuddalore in kilometres.
3. Converting metre into centimetre
Since 1 m = 100 cm, to change metres into centimetres, multiply by 100.
- 5 m = 5 × 100 = 500 cm
- 13 m = 13 × 100 = 1300 cm
When the measurement has both parts, convert the metres and then add the centimetres:
4 m 35 cm = (4 × 100) + 35 = 400 + 35 = 435 cm
4. Converting centimetre into metre
Going the other way, divide by 100.
- 600 cm = 600 ÷ 100 = 6 m
- 3600 cm = 36 m
If it does not divide evenly, the remainder stays as centimetres:
647 cm = 6 m 47 cm, because 600 cm makes 6 m and 47 cm is left over.
The quick way: in a centimetre figure, the last two digits are the centimetres and whatever comes before them is the metres.
5. Adding measurements
Write metres under metres and centimetres under centimetres, then add each column. If the centimetres reach 100 or more, carry 1 metre across.
Add 34 m 91 cm and 25 m 42 cm:
| m | cm | |
|---|---|---|
| 34 | 91 | |
| + | 25 | 42 |
| 60 | 33 |
The centimetres gave 91 + 42 = 133 cm. That is 1 m and 33 cm, so 33 stays and 1 m is carried: 34 + 25 + 1 = 60 m. The answer is 60 m 33 cm.
6. Subtracting measurements
Same columns. If the top centimetres are too small, borrow 1 metre and turn it into 100 cm.
Subtract 18 m 24 cm from 48 m 36 cm. No borrowing is needed here:
48 m 36 cm − 18 m 24 cm = 30 m 12 cm
Now one that does need borrowing. Subtract 54 m 75 cm from 73 m 44 cm. Since 44 is less than 75, borrow a metre from 73, leaving 72 m and making the centimetres 144:
144 − 75 = 69 cm, and 72 − 54 = 18 m. So the answer is 18 m 69 cm.
7. Problems about length and distance
From the book. The distance between two coconut trees is 70 m 35 cm. Suthan walks from the first tree to the second and back again. How far has he walked?
He covers the distance twice:
70 m 35 cm + 70 m 35 cm = 140 m 70 cm
From the book. Agathiya bought 70 m 42 cm of wire to fence his garden and used only 43 m 51 cm. How much is left?
Since 42 cm is smaller than 51 cm, borrow a metre: 142 − 51 = 91 cm, and 69 − 43 = 26 m. He has 26 m 91 cm left.
8. Estimation
To estimate is to give a sensible answer without measuring. It is a skill worth having, because it catches silly mistakes.
The book offers a useful body ruler: one metre is roughly the distance from your shoulder to your foot when you stand straight. Knowing that, you can guess a door at about 2 m and a blackboard at about 3 m before you check.
Always estimate first, then measure. If the two are wildly apart, one of them is wrong.
9. Weight and capacity
Length is not the only thing we measure. Weight and capacity follow exactly the same pattern of a big unit made of 1000 small ones.
| Relationship | Used for |
|---|---|
| 1 kilogram (kg) = 1000 gram (g) | Rice, vegetables, your own weight |
| 1 litre (L) = 1000 millilitre (mL) | Milk, oil, petrol, medicine |
| 1 kilometre (km) = 1000 metre (m) | Distance between towns |
The conversion rule is the same every time: big unit into small unit, multiply; small unit into big unit, divide.
- 3 km = 3 × 1000 = 3000 m
- 4 kg 250 g = (4 × 1000) + 250 = 4250 g
- 2500 mL = 2500 ÷ 1000 = 2 L 500 mL
A warning about halves. Half a metre is 50 cm, not 5 cm, because a metre holds 100 cm. So 1.5 m = 1 m 50 cm. In the same way half a litre is 500 mL and half a kilogram is 500 g.
10. Worked examples
Example 1. Convert 7 m into centimetres.
Solution: 7 × 100 = 700 cm.
Example 2. Convert 5 m 9 cm into centimetres.
Solution: (5 × 100) + 9 = 500 + 9 = 509 cm.
Example 3. Convert 1300 cm into metres.
Solution: 1300 ÷ 100 = 13 m.
Example 4. Add 4 m 75 cm and 3 m 18 cm.
Solution: 75 + 18 = 93 cm (no carry). 4 + 3 = 7 m. Answer: 7 m 93 cm.
Example 5. Deenu bought 15 m 43 cm of shirt material and 23 m 94 cm of trouser material. Find the total.
Solution: 43 + 94 = 137 cm = 1 m 37 cm. Carry the metre: 15 + 23 + 1 = 39 m. Total = 39 m 37 cm.
Example 6. Subtract 20 m 48 cm from 96 m 32 cm.
Solution: 32 is less than 48, so borrow: 132 − 48 = 84 cm, and 95 − 20 = 75 m. Answer: 75 m 84 cm.
Example 7. Raju had a 20 m ribbon and used 13 m 25 cm for a project. How much is left?
Solution: Write 20 m as 19 m 100 cm. Then 100 − 25 = 75 cm and 19 − 13 = 6 m. Answer: 6 m 75 cm.
11. Practice
- How many centimetres are there in 1 metre?
- Convert 16 m into centimetres.
- Convert 4000 cm into metres.
- Convert 7 m 35 cm into centimetres.
- Add 25 m 53 cm and 18 m 24 cm.
- Subtract 36 m 24 cm from 63 m 47 cm.
- A fisherman's two nets are 23 m 43 cm and 25 m 63 cm long. Find the total length.
- Convert 1700 cm into metres.
- Roughly how long is one metre, measured on your own body?
- Convert 3 km into metres.
- Convert 4 kg 250 g into grams.
- Convert 2500 mL into litres and millilitres.
- Write 1.5 m in metres and centimetres.
12. Answers
- 100 cm.
- 16 × 100 = 1600 cm.
- 4000 ÷ 100 = 40 m.
- 700 + 35 = 735 cm.
- 53 + 24 = 77 cm; 25 + 18 = 43 m. Answer 43 m 77 cm.
- 47 − 24 = 23 cm; 63 − 36 = 27 m. Answer 27 m 23 cm.
- 43 + 63 = 106 cm = 1 m 6 cm. Carry: 23 + 25 + 1 = 49 m. Total 49 m 6 cm.
- 17 m.
- About the distance from your shoulder to your foot when standing straight.
- 3 × 1000 = 3000 m.
- 4000 + 250 = 4250 g.
- 2 L 500 mL.
- 1 m 50 cm — half a metre is 50 cm, not 5 cm.
13. Summary
- 10 mm = 1 cm, 100 cm = 1 m, 1000 m = 1 km; 1 mile is about 1 km 600 m.
- Metres to centimetres: multiply by 100. Centimetres to metres: divide by 100.
- In a centimetre figure, the last two digits are centimetres and the rest are metres, so 647 cm = 6 m 47 cm.
- Add and subtract in two columns, m under m and cm under cm.
- Carry 1 m when the centimetres reach 100; borrow 1 m as 100 cm when they are too small to subtract.
- A there-and-back journey covers the distance twice.
- Estimate before measuring; one metre is about shoulder height to the floor.
