Measurement and Unit Conversions

Imagine telling your friend the MRT ride is "1500" long. 1500 what? Centimetres? Metres? Without a unit, a number means nothing. Measurement is all about pairing a number with the right unit — and being able to swap one unit for another without losing the amount. Get this right and a whole family of PSLE word problems opens up.

Parents: let your child predict whether each conversion needs multiply or divide before they reveal. The "Method tip" boxes name the exact step a PSLE marker rewards in the working.

By the end you'll be able to name the metric units, convert between them, read compound units, and solve measurement word problems. Let's measure up.

The metric units you must know

In Singapore we use the . For length the units are kilometre (km), metre (m), centimetre (cm). For mass they are kilogram (kg) and gram (g). For volume of liquid they are litre and millilitre (ml).

The numbers that link them are friendly. 1 km = 1000 m, 1 m = 100 cm, 1 kg = 1000 g, and 1 litre = 1000 ml. Learn these four facts and every conversion is just a multiply or a divide.

🤔 Predict first: How many centimetres are there in 1 metre?

Bigger to smaller: multiply

Here is the one idea that makes every conversion easy. Going from a bigger unit to a smaller unit, you multiply, because it takes many small units to make one big one.

For example, 3 m to cm: there are 100 cm in each metre, so 3 × 100 = 300 cm. Or 2 kg to g: 2 × 1000 = 2000 g. The number gets bigger, which makes sense — a centimetre is tiny, so you need lots of them.

🤔 Predict first: Convert 4 litres to millilitres.

Smaller to bigger: divide

Going the other way — from a smaller unit to a bigger unit — you divide, because many small units squash into one big one.

For example, 500 cm to m: 500 ÷ 100 = 5 m. Or 3000 g to kg: 3000 ÷ 1000 = 3 kg. The number gets smaller, which is the clue you are dividing correctly.

🤔 Predict first: Convert 2500 ml to litres.

Show the worked solution

Check your answer makes sense: 2500 ml is only $2\frac{1}{2}$ full bottles, so 2.5 litres is reasonable, while 250 litres would be a whole tank.

Measurement lab: same amount, two readings

Predict first: 2 km 350 m is how many metres?

Decimal-form conversions in both directions

The same four links work when the amount is written as a decimal. This is . First calculate with the conversion factor. Then write the equivalent measurement with its units. The arithmetic changes the number's form; the amount itself stays the same.

🤔 Predict first: To change 2.375 km into metres, which numerical operation should you start with?

Kilometres and metres

The link is 1 km = 1000 m. For the larger-to-smaller direction, multiply; for the reverse direction, divide.

km and m: both directions

2.375\times1{,}000=2{,}375

2{,}375\div1{,}000=2.375\text{ (reverse check)}

So $2.375\,\text{km}=2{,}375\,\text{m}$ . In the other direction, $2{,}375\,\text{m}=2.375\,\text{km}$ .

Metres and centimetres

The link is 1 m = 100 cm.

m and cm: both directions

4.08\times100=408

408\div100=4.08\text{ (reverse check)}

So $4.08\,\text{m}=408\,\text{cm}$ . In the other direction, $408\,\text{cm}=4.08\,\text{m}$ .

Kilograms and grams

The link is 1 kg = 1000 g.

kg and g: both directions

1.625\times1{,}000=1{,}625

1{,}625\div1{,}000=1.625\text{ (reverse check)}

So $1.625\,\text{kg}=1{,}625\,\text{g}$ . In the other direction, $1{,}625\,\text{g}=1.625\,\text{kg}$ .

Litres and millilitres

The link is 1 litre = 1000 ml.

l and ml: both directions

2.4\times1{,}000=2{,}400

2{,}400\div1{,}000=2.4\text{ (reverse check)}

So $2.4\,\text{l}=2{,}400\,\text{ml}$ . In the other direction, $2{,}400\,\text{ml}=2.4\,\text{l}$ .

Practice the decimal directions

Practice 1. Convert 0.75 km to metres. Then use the reverse operation to check.

Show the solution

Practice 1

0.75\times1{,}000=750

750\div1{,}000=0.75\text{ (check)}

Therefore $0.75\,\text{km}=750\,\text{m}$ .

Practice 2. Convert 360 cm to metres. Then use the reverse operation to check.

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Practice 2

360\div100=3.6

3.6\times100=360\text{ (check)}

Therefore $360\,\text{cm}=3.6\,\text{m}$ .

Practice 3. A recipe uses 1.25 kg of flour. How many grams is that? Check by converting back.

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Practice 3

1.25\times1{,}000=1{,}250

1{,}250\div1{,}000=1.25\text{ (check)}

Therefore $1.25\,\text{kg}=1{,}250\,\text{g}$ .

Compound units like 2 m 5 cm

Sometimes a length is written in two units at once, like 2 m 5 cm. This is a . To turn it all into the smaller unit, change the big part first, then add the small part.

So 2 m 5 cm becomes 2 × 100 = 200 cm, then add the 5 cm, giving 205 cm. Be careful: 2 m 5 cm is not 250 cm. The 5 is only 5 cm, so you add just 5, not 50.

🤔 Predict first: Write 3 kg 40 g in grams.

Show the worked solution

3 kg = 3000 g, plus 40 g = 3040 g.

Measurement word problems

Most PSLE measurement questions hide a conversion inside a word problem. The trick is to make all the units the same first, then add, subtract, multiply or divide as normal.

For example: a rope is 2 m long and Mei cuts off 45 cm. How much is left? Change 2 m to 200 cm, then 200 − 45 = 155 cm. If you forget to convert, you would wrongly try 2 − 45, which makes no sense.

🤔 Predict first: A bottle holds 1 litre 200 ml of water. 350 ml is poured out. How much is left?

Watch out — these are easily mixed up

Quick recap

🎯 Mastery check

Answer all 8 — your progress is saved on this device.

Convert 7 m to centimetres.
Convert 4000 g to kilograms.
Write 5 m 8 cm in centimetres.
A jug holds 2 litres of juice. 750 ml is poured into a cup. How much juice is left in the jug?
Convert 3500 m to kilometres.
A box of 4 identical books weighs 1 kg 200 g in total. What is the mass of one book?
Convert 1.625 kg to grams.
Convert 408 cm to metres.