Choose the correct unit
A ribbon might be measured in centimetres, while a playground might be measured in metres.
ribbon or pencil
room or playground
thickness
Use the size of the object to choose the best unit.
Year 3
Measure, compare, add and subtract lengths, mass and volume or capacity
Use standard units to measure, compare and combine lengths, mass and capacity accurately.
Lesson overview
Measurement
Different measurements use different units. Length can be measured in millimetres, centimetres or metres. Mass can be measured in grams or kilograms. Volume and capacity can be measured in millilitres or litres.
Compare and add
125 cm + 40 cm = 165 cm.
Only add measurements when they use the same unit.
Mass and capacity
A bag of flour might be 1 kg. A water bottle might hold 750 ml.
flour
water bottle
Mass tells us how heavy. Capacity tells us how much a container holds.
Facts to know
These facts help when a question uses two different units.
Write both measurements in the same unit before you add, subtract or compare.
Read a scale
A scale has marks that are the same distance apart. Follow the five steps to read the blue marker.
Slide the scale sideways to see all of it.
Ruler
0 cm to 10 cm
The marks numbered 5 and 7 are 2 cm apart over 2 steps, so one step is 1 cm. The marker is 1 step after 5 cm, so it shows 6 cm.
Read a mass scale and a capacity scale
Mass scales use grams (g) and capacity scales use millilitres (ml). Read them in the same way.
Slide the scale sideways to see all of it.
Mass scale
0 g to 500 g
Slide the scale sideways to see all of it.
Measuring jug
0 ml to 1000 ml
On the mass scale, 200 g and 400 g are 200 g apart over 2 steps, so one step is 100 g and the marker shows 300 g. On the jug scale, 250 ml and 750 ml are 500 ml apart over 2 steps, so one step is 250 ml and the marker shows 500 ml.
Worked example
A string is 38 cm long. Another string is 24 cm long. What is the total length?
- Both lengths are measured in centimetres.
- Add 38 and 24.
- 38 + 24 = 62.
- So the total length is 62 cm.
Try it
Check the unit first, then compare, add or subtract the measurements carefully.
Question 1
Which unit is best for the height of a door?
Question 2
Which is heavier?
Question 3
48 cm + 12 cm =
Question 4
Which unit is best for a jug of juice?
Question 5
Which statement is correct?
Common mistakes
Watch for these when working through the lesson.
- Mixing units without noticing.
- Choosing litres for a very small amount.
- Adding measurements but forgetting to write the unit in the answer.
Related topics
These ideas fit closely with this lesson.
- Measure the perimeter of simple 2-D shapes.
- Add and subtract amounts of money to give change, using both pounds and pence.