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 metric system. 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.

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.

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.

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 compound unit. 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.

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.

Watch out — these are easily mixed up

Quick recap

🎯 Mastery check

Answer all 6 — your progress is saved on this device.

1. Convert 7 m to centimetres.

   70 cm700 cm7000 cm

2. Convert 4000 g to kilograms.

   4 kg40 kg0.4 kg

3. Write 5 m 8 cm in centimetres.

   58 cm580 cm508 cm

4. A jug holds 2 litres of juice. 750 ml is poured into a cup. How much juice is left in the jug?

   1250 ml1750 ml1350 ml

5. Convert 3500 m to kilometres.

   35 km3.5 km350 km

6. A box of 4 identical books weighs 1 kg 200 g in total. What is the mass of one book?

   300 g400 g250 g

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