P5 Maths | Unit Conversion Lesson-Prep Guide (for tutors) | 45 minutes from the conversion table to a three-step check

2026-11-08

Tutor lesson-prep guides · Primary Maths

Conversion questions are never lost on arithmetic, they are lost on whether to multiply or divide. One sentence settles it: a bigger unit into a smaller one makes the number bigger.

Objectives — Students can: (1) state the relationships between kilometres, metres, centimetres and millimetres and convert in both directions; (2) convert kilograms and grams, litres and millilitres, handling decimal points correctly; (3) convert minutes and hours, including 45 minutes as 0.75 hours; (4) use the direction rule to choose multiplication or division and attach the right unit.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Decimal multiplication	2.5 times 1000	2500
Decimal division	45 divided by 60	0.75
Unit knowledge	How many metres in a kilometre?	1,000 metres
Time knowledge	How many minutes in an hour?	60 minutes

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Opening: a pencil	Ask which unit measures a pencil and which measures a classroom	Name centimetres and metres and say why
8-17 min	The length table	Write km, m, cm, mm with steps of 1000, 100 and 10	Six length conversions
17-26 min	Mass and capacity	Show 2.5 kg as 2500 g and 3 L as 3000 mL	Six questions, watching the decimal point
26-34 min	Time conversion	Show 45 minutes as 0.75 hours, starting from the fraction	Five time conversions
34-43 min	Mixed and applied	Questions mixing units, so unify before calculating	Four word problems
43-45 min	Exit check	Three quick oral questions	Independent answers

03 Teaching script (read it out)

Scene 1: One sentence decides multiply or divide

Students convert 2.5 kg to grams by dividing, get 0.0025 and do not notice the absurdity. Teach the direction rule: a bigger unit into a smaller one always makes the number bigger, so multiply, and divide the other way.
Back it with common sense: 2.5 kilograms of rice cannot weigh 0.0025 grams. Asking whether the number is plausible beats memorising formulas.

Students do: say “big to small, so multiply” before each calculation.

Scene 2: Build the table by hand once

Students copy a table and cannot recall it because they never took part in making it. Work down from a ruler to a kilometre, writing 1 km = 1000 m, 1 m = 100 cm, 1 cm = 10 mm on the board with an arrow for multiply and one for divide at every step.
Then have them rebuild it on blank paper and correct it on the spot. A table you built survives far longer than one you copied ten times.

Students do: rebuild the table from memory, arrows included.

Scene 3: Time is not decimal, so treat it separately

Students treat time as decimal and write 45 minutes as 0.45 hours. Start from the fraction: 45 minutes is 45/60 of an hour, which reduces to 3/4, that is 0.75.
Add the framing that time is the one family that is not base ten while length, mass and capacity use 10, 100 and 1000. Marking the exception off separately stops it contaminating the other conversions.

Students do: write 15, 30 and 45 minutes as both fractions and decimals of an hour.

04 Question types and worked steps

Type	Example	Steps	Answer
Length	3.2 m equals how many cm?	Big to small, multiply, 1 m = 100 cm	320 cm
Mass	2.5 kg equals how many g?	Multiply by 1000, move the point three places	2500 g
Capacity	3 L equals how many mL?	Multiply by 1000	3000 mL
Time	45 minutes equals how many hours?	Divide by 60, write the fraction first	0.75 hours

Sanity check (teach this): Sanity checks: converting a larger unit to a smaller one always increases the number; the decimal point moves by the same number of places as the conversion factor; time divides by 60 and never by 100; answers carry units; applied questions unify units before adding or subtracting; results match everyday sense, since a book does not weigh 500 kilograms.

05 Independent practice (3 questions)

Question	Answer	What to watch
Six length items	km, m, cm and mm mixed, both directions	Check the direction chosen
Six mass and capacity items	Including decimals and one small-to-big direction	Check decimal places
Five time and applied items	Three time, two mixing two units	Check units were unified first

06 Common mistakes and what to say

Mistake	Student writes	Say this
Direction reversed	2.5 kg answered as 0.0025 g	Rice weighing 2.5 kilograms cannot be 0.0025 grams. Big to small makes the number bigger, so multiply.
Decimal point moved wrongly	3.2 m answered as 32 cm	One metre is 100 centimetres, so the point moves two places, not one.
Time treated as decimal	45 minutes answered as 0.45 hours	45 minutes is 45/60, that is 3/4 or 0.75 hours; time is not base ten.
Adding mixed units directly	1 m 20 cm plus 80 cm answered as 100 cm	Convert first: 120 plus 80 is 200 cm, which is 2 metres.

07 Homework (paper, section and question numbers)

Primary maths chapter index — All chapters for this subject
Lesson-prep guides index — Similar chapters in other subjects
Redo this chapter — Redo five time conversions, fraction first then decimal

08 Exit check (last 5 minutes)

Question	Answer	Decision
Big to small	Multiply	Dividing means going back to the second stage
Centimetres in a metre	100	Answering 10 means going back to the third stage
45 minutes in hours	0.75	Answering 0.45 means going back to the fifth stage
Mixed units first	Unify them	Adding directly means going back to the sixth stage

09 Tutor FAQ

Must the whole table be memorised?

Four core factors are enough: 1000, 100, 10 and 60. Everything else can be derived, which is more reliable than a long memorised list.

Decimals or fractions first for time?

Use fractions to explain the reasoning, then convert to decimals; length and mass go straight to multiplication and point shifting.

What if word problems are hard to read?

Have students circle every number with a unit, convert them all to one unit, then write the expression. Twenty extra seconds of reading halves the errors.

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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.