Primary 3 Maths | Length and Distance: Tutor Prep Guide | 45 minutes from measuring to kilometre conversion problems

2026-11-08

Tutor lesson-prep guides · Primary Maths

Length questions are almost never lost on arithmetic. They are lost on units: a child computes 2 + 350 = 352 and writes 352 metres. So fix 1 kilometre = 1000 metres first, before any calculation.

Objectives — Pupils can: (1) measure an object accurately to the nearest millimetre; (2) state the relationship between kilometres, metres, centimetres and millimetres and convert between them; (3) rewrite 2 km 350 m as a single unit before calculating; (4) solve two-step distance problems with the unit written in the answer.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Reading a ruler	How many small gaps between 0 and 1 on a ruler	Ten, one millimetre each
Adding three-digit numbers	640 + 520	1160
Multiplication with carrying	25 x 4	100
Comparing lengths	Which is longer, 1.5 m or 160 cm	160 cm

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0–8 min	Starter: measure a pencil	Pupil measures and reads the ruler aloud in centimetres and millimetres	Reads 14 cm 5 mm
8–17 min	Four units in a row	Order mm, cm, m, km on the board with the step multiplier written between each	States each multiplier
17–26 min	Conversion practice	Model 2 km 350 m to 2350 m, then the reverse: 1750 m to 1 km 750 m	Both directions correct
26–35 min	Comparing lengths	Put lengths in different units side by side and convert first	Says units first, comparison second
35–43 min	Distance problems	Home to school 640 m, school to library 1110 m, ask for the total	Writes the working with units
43–45 min	Exit check	Three quick oral questions	Answers independently

03 Teaching script (read it out)

Scene 1: A unit is part of the working, not decoration

Pupils write 2350 with no unit, or add kilometres straight to metres. Show the consequence: 2 km plus 350 m becomes 352 if you refuse to convert, and nobody believes 352.
Make the conversion visible: write 2 km = (2000 m) on the line above the calculation instead of doing it in the head. Mark only answers that carry units.

Pupil does: line one is the conversion, line two is the arithmetic.

Scene 2: Big unit to small unit means multiply

Pupils divide by 1000 when converting kilometres to metres and the answer collapses. Do not rely on a rhyme; use the physical sense: a kilometre is long, so measured in metres it must be a big number, therefore multiply.
Give a self-check that needs no memory: after converting, the number should have grown. If it shrank, the direction was wrong.

Pupil says: after converting, will the number grow or shrink, and why.

Scene 3: Measure from zero, not from the end of the ruler

Many rulers have a gap between the plastic edge and the zero mark, so pupils measure a few millimetres short every time. Line the object up with zero.
Second check: measuring a pencil from 3 cm to 17 cm gives 17 minus 3 = 14 cm. Use this to establish end minus start as the general rule.

Pupil does: measure twice, once from zero and once from 3 cm, and get the same answer.

04 Question types and worked steps

Type	Example	Steps	Answer
Measuring	Measure the long side of a textbook	Line up with zero, read to the millimetre	About 26 cm 0 mm
Conversion	How many metres is 2 km 350 m	Multiply the big unit by 1000, then add	2350 m
Comparing	1.5 m against 160 cm	Convert both to the same unit	160 cm is longer
Distance problem	Home to school 640 m, school to library 1110 m	Draw a bar model, then add or subtract	Total 1750 m, library 470 m further

Sanity check (teach this): Reasonableness checks: every answer carries a unit; conversion moves the number in the sensible direction (kilometres to metres grows); addition and subtraction happen only within one unit; measurements match the real object (a book is not 26 metres); the answer to a word problem is restated in a sentence; 1 km = 1000 m, 1 m = 100 cm, 1 cm = 10 mm, never reversed.

05 Independent practice (3 questions)

Question	Answer	What to watch
5 conversion questions	Both directions, one with a remainder	Checks direction and how the remainder is written
4 measuring tasks	Three real objects, one measured from a non-zero start	Checks end minus start
4 distance problems	Two additions, two subtractions, all with units	Checks units and the closing sentence

06 Common mistakes and what to say

Mistake	Student writes	Say this
No unit in the answer	Writes 352 and stops	“352 what? Metres or kilometres? An answer without a unit tells nobody anything.”
Conversion the wrong way	2 km written as 0.002 m	“A kilometre is long. In metres that must be a big number, and yours got smaller.”
Measuring from the edge	Result a few millimetres short	“Line up with zero, not with the edge. From 3 cm to 17 cm is 14 cm.”
Adding different units directly	2 km + 350 m = 352	“Kilometres and metres cannot be added as they stand. Convert first.”

07 Homework (paper, section and question numbers)

Primary 1 to 6 Maths test papers with answers — Do the P3 length questions, writing the conversion first
Primary Maths chapter index — All chapters in the subject
Redo this chapter — Redo four word problems, drawing a bar model each time

08 Exit check (last 5 minutes)

Question	Answer	Decision
How many metres in a kilometre	1000	Wrong means going back to the third stage
What is wrong with measuring from the ruler edge	The edge is not always zero, so it reads short	Saying nothing is wrong means back to the second stage
What comes before adding lengths	Convert to the same unit	Saying add directly means back to the fifth stage
How many metres is 2 km 350 m	2350 m	Answering 352 means back to the fourth stage

09 Tutor FAQ

Must the conversion table be memorised?

Yes, but build it on the board with the pupil ordering millimetres to kilometres and writing each multiplier by hand.

Accurate with a ruler but wrong on word problems

The problem is reading, not measuring. Make them draw a bar model first and label every part before choosing to add or subtract.

Should 0.35 km be taught here?

Only as the same distance written differently. Decimal arithmetic belongs to P4; the point now is that both mean the same stretch.

Find a tutor — free Tutor Registration →

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.