Primary 2 Maths | Length in cm and m Guide | Unit Choice, Three-Step Measuring, Estimate Check

2026-11-09

Tutor lesson-prep guides · Primary Maths

A child writes 30 and calls it finished, because he measured a number, not a length. The unit is part of the answer: 30 centimetres and 30 metres differ by a factor of 100.

Objectives — Students can: (1) choose centimetres or metres for an object and give the reason, short objects in centimetres, long ones in metres; (2) measure in three steps: line the object up with zero, lay the ruler along the edge, read the end and write the unit; (3) estimate before measuring and state how many centimetres the estimate was out.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Numbers to 100	Read the marks on a ruler	Finds 0, 10 and 20
Comparing lengths	Compare two pencils with ends aligned	Says longer, shorter, same
Unit sense	Which is longer, 1 metre or 1 centimetre	Metres for long things

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Unit diagnosis	Two answers with numbers only; which is finished	Say that a unit is needed
5-15 min	Choose a unit	Short uses cm, long uses m	One reason per object
15-25 min	Three steps	Zero, edge, read the end	Measure one object per step
25-33 min	Estimate and error	Estimate, measure, write the gap	Estimate three, record the gap
33-40 min	Convert and compare	1 m = 100 cm, compare two lengths	Three items with units
40-45 min	Exit check	Three questions collected and marked	Work independently

03 Teaching script (read it out)

Scene 1: The unit is part of the answer

A child hands in 30 and cannot say 30 what. The arithmetic is fine; he treats measuring as reading a number.
Be explicit: a measurement is a number plus a unit. The same number with a different unit changes the length by a factor of 100: 30 centimetres is about a school ruler, 30 metres is a corridor.
So the last move on every answer is one question: which unit did I use, and is it sensible? A pencil at 17 metres is wrong on sight, because metres are for long things.

Students do: say number plus unit for every object they measured, and give the reason for the unit.

Scene 2: Three steps, and every one of them matters

Ruler errors come in three shapes, so the routine has three steps:
1. Line up with zero: put one end of the object at the 0 mark, not at the edge of the ruler. Aligning with the edge adds the blank strip at the ruler head.
2. Lay the ruler along the edge: a tilted ruler reads high, because you are measuring the slant.
3. Read the far end and write the unit at once.
Teach all three together. When a child misses one, redo that step, not the whole question.

Students do: measure a pencil, an eraser and a desk, naming the step they used each time.

Scene 3: Estimate first, then explain the error

Estimating is not guessing; it uses known lengths as rulers: a pencil about 17 cm, a desk about 60 cm, a classroom door about 2 m.
The routine: write the estimate, measure, then write the gap. A gap within 5 cm is reasonable at this level; a large gap needs an explanation, usually a metre read as a centimetre.
This step builds unit sense. A child who can estimate has understood the difference between centimetres and metres.

Students do: estimate three objects, measure them, and write estimate, measurement and gap.

04 Question types and worked steps

Type	Example	Steps	Answer
Choose the unit	Fill in centimetres or metres	Ask whether it is longer than a ruler	A number alone is unfinished
Read the ruler	Read the length from a picture	Zero, edge, far end	Read the end, not the count of gaps
Estimate	Estimate, measure, write the gap	Use a known length as the reference	A big gap needs a reason
Convert and compare	1 m = 100 cm, compare two lengths	Convert before comparing	Never compare across units

Sanity check (teach this): Sanity check: 1 m = 100 cm, and the direction of conversion must stay consistent; measuring is always three steps, zero, edge, far end plus unit, and skipping one produces a skew; a 5 cm estimate gap is the Primary 2 range; lengths must share a unit before comparison, or children compare bare numbers; the exit check is 300 seconds for three questions, which is doable independently at this level.

05 Independent practice (3 questions)

Question	Answer	What to watch
Unit choice	Four objects, cm or m	All four justified
Measuring	Three objects with units	Zero aligned, unit written
Estimating	Two objects, estimate then measure	Gap stated in cm
Comparing	Convert then compare two lengths	Same unit on both sides

06 Common mistakes and what to say

Mistake	Student writes	Say this
Numbers without units	Writing 30 and stopping	30 what? Centimetres or metres? Those differ by 100 times, so write it.
Starting at the ruler edge	Measurement comes out too long	Did you start at 0 or at the edge? Line it up with 0 and read again.
Tilting the ruler	Reading longer than the object	Keep the ruler along the edge. Tilted, you measured the slant, not the length.
Comparing across units	Treating 2 m as smaller than 150 cm	Convert first. How many centimetres is 2 metres? Then compare.

07 Homework (paper, section and question numbers)

Primary maths chapter index — All chapters by level
Teaching guide index — Same structure, other subjects
Redo this chapter — Questions 1 to 4 again, with units every time

08 Exit check (last 5 minutes)

Question	Answer	Decision
How many steps in measuring	3: zero, edge, read and write the unit	Two means act two again
How many centimetres in a metre	100	Wrong means act three again
What comes before comparing 2 m with 150 cm	Convert to one unit	Compare directly means act three again

09 Tutor FAQ

Why not teach millimetres first?

Primary 2 needs two scales only. Three at once turns units into a chant and costs the sense of length.

Must the gap be written down?

Yes. An estimate with no gap never shows the child how far off the unit sense is, and nothing improves.

Can a tape measure replace a ruler?

It can, but it must have a clear zero. Some tapes measure from the case edge, which ruins the zero step.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.