The idea in brief. A compound measurement is the sum of whole larger units and the extra smaller units, using the correct unit relationship. Use 100 centimetres in a metre, but 1000 in the other three unit pairs. Keep small remainders small. In the lesson. Let's remember the four pairs of units. Today, we will change a measurement in two units into the smaller unit, and back again. Change the whole big units first: one hundred for metres, one thousand for the others. The extra small units stay as they are.
Lesson video, with the working
Converting compound measurements
4 min 43 s
The film teaches 3 questions in turn. Each question shows first; its working is written as the film explains it.
This page teaches converting a measurement in two units into the smaller unit, and back again, for the four P3 pairs: kilometres and metres, metres and centimetres, kilograms and grams, and litres and millilitres.
What your child learns here
This page teaches converting a measurement in two units into the smaller unit, and back again, for the four P3 pairs: kilometres and metres, metres and centimetres, kilograms and grams, and litres and millilitres. It follows the P3 outcome on converting measurements in compound units, with numbers kept easy to work with.
Recall the four relationships: 1 km = 1000 m, 1 m = 100 cm, 1 kg = 1000 g, and 1 litre = 1000 ml.
Change two units to the smaller unit: convert the whole big units, then add the extra small units.
Change the smaller unit to two units: take out the whole big units and leave the rest as small units.
Handle small leftovers with zeros, such as 4 kg 8 g written as 4008 g.
Does the answer make sense?
In the smaller unit, the big units show up as the front digits: 3 kg and some grams must come to three thousand and something grams. Going the other way, your child's leftover small part must be under one big unit, so less than 100 cm, 1000 m, 1000 g or 1000 ml.
How to practise these questions
The Basic questions practise converting compound measurements into the smaller unit. The Intermediate questions add converting the other way, from the smaller unit into two units. The Challenge questions include a small second part, under 100 g or 100 ml, so a 0 must hold a place. If converting stalls, view the solution film for that question and try once more later from Revision. A multiple-choice Basic question asks which measurement is equal to a length given in metres and centimetres, and its solution is written working with no film.
Questions parents ask
Which unit conversions does my P3 child need? Four pairs: kilometres and metres, metres and centimetres, kilograms and grams, and litres and millilitres. The Primary 3 syllabus asks for converting a measurement in compound units to the smaller unit, and back, with numbers that are easy to work with.
Why is it 100 for centimetres but 1000 for the other units? That is how the metric units are defined: 1 m is 100 cm, while 1 km is 1000 m, 1 kg is 1000 g and 1 litre is 1000 ml. Your child simply needs to remember which pair uses 100.
Does P3 unit conversion include decimals like 1.5 kg? No. The Primary 3 syllabus works with compound units such as 1 kg 500 g rather than decimals. Decimal measurements come later in the syllabus.
