Mathematics K–6 · Years 3–4

Estimate, then measure: metres, centimetres and millimetres

Measurement and space

PracticalLow risk

The idea

Metric units are fixed sizes that everyone shares, and an estimate improves when it is checked against a measurement and the error is recorded.

What you need

a 1 m rule marked in centimetres and a 5 m tape measure
a 30 cm ruler marked in millimetres
a 1 m length of string
a recording table with columns object, estimate, measurement, difference

How to do it

Hold your arms apart to show what you think 1 m is, then check against the metre rule. Do the same for 10 cm with finger and thumb, and 1 mm with the ruler.
Choose eight objects in the room (door height, desk length, book width, pencil length, a paperclip). Estimate each in the best unit, then measure and record the difference.
Measure the long side of an A4 sheet in millimetres and the room length in metres and centimetres. Write each measurement two ways (for example 2 m 40 cm and 240 cm).
Repeat the estimating with eight new objects and compare the differences with the first round.
Lay the metre rule and ten 10 cm strips side by side, then count millimetre marks in one centimetre, to record 1 m = 100 cm and 1 cm = 10 mm.

What you should see

An A4 sheet measures 297 mm by 210 mm (the ISO 216 size), and 1 m = 100 cm = 1000 mm, so a length can be written two ways, such as 2 m 40 cm and 240 cm. Comparing the two rounds shows whether the differences between estimate and measurement have shrunk; a child who measured their own hand span, arm span and stride in the first round has known lengths to estimate from. The same object measured twice with the tape gives readings that agree closely; readings taken from the 1 cm mark instead of 0 come out 1 cm too long, and the pair finds the fault by re-measuring.

What changes

What you change: the round of estimating (first or second)
What you measure: the difference between estimate and measurement
What you keep the same: the same units for the same kind of object
measurements taken from the zero mark
the same measurer for both rounds

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Measuring from the end of the ruler rather than the zero mark gives the same answer.
Centimetres and millimetres are different names for the same size.

Safety card

Low riskLearners carry it out

Hazards

tape measures retracting fast
measuring near doors

Controls

retract tapes slowly; keep doors propped while measuring

Note

No chemicals or heat are used, so the NSW Department of Education Chemical Safety in Schools package is not needed; ordinary classroom supervision under the school's usual risk management.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Mathematics K-10 Syllabus (2022), NSW Education Standards AuthorityMA2-GM-02 MAO-WM-01
Australian Curriculum v9AC9M3M01AC9M3M02AC9M4M01

Sources

The pages the author read to write this activity.

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