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NSW SelectiveYear 7 entry

Unit conversions for the Selective maths test

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Unit conversions in the Selective maths test follow one golden rule: to a smaller unit, multiply; to a bigger unit, divide. Your child changes every amount to the same unit first, then calculates and answers in the unit asked.

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The full lesson is in the SelectiveReady app. A free account opens How and Why Authors Write; this lesson needs a paid plan.

Updated 16 October 2026 · Facts checked 12 October 2026

Measurement & Unit Conversions (lesson preview)

A short preview from the SelectiveReady lesson “Measurement & Unit Conversions”, on unit conversions for the NSW Selective test.

Published:

Read the video transcript

Are we going up or down the staircase? We're starting with litres, and we wanna get to millilitres. Litres are the big unit, right? Millilitres are the tiny ones, so we're going from big to small. And what does our golden rule tell us to do when we go from a big unit to a small one? Exactly, we multiply. It just makes sense. We know we're gonna end up with a much bigger number, 'cause we're breaking that 1.5 litres into loads of tiny millilitre pieces. Okay, next up, you just need to know the conversion factor. For litres and millilitres, just like for kilometres and metres, the magic number is 1,000. There are 1,000 millilitres in one litre. So we simply calculate 1.5 times 1,000. And there you have it. 1.5 times 1,000 is 1,500. So 1.5 litres is equal to 1,500 millilitres. See, the system works. You just follow the steps and you get the right answer.

Measurement & Unit Conversions is a full lesson in the SelectiveReady app. A free account opens one lesson, How and Why Authors Write; the other lessons need a paid plan1SelectiveReady · 13 October 2026 · Published. The preview plays here with English and Chinese captions.

2

What will your child learn in this lesson?

Your child learns that converting units works like changing money: swap one dollar and you get a hundred cents, more pieces of a smaller thing. They'll be able to move up and down the metric staircase, choose multiply or divide without guessing, compare amounts in different units and read a scale.

The clip changes a bottle's litres into millilitres using the golden rule. The full lesson adds the whole staircase, three traps and a checklist.

3

How do units show up in the Selective maths test?

Units run through the whole Mathematical Reasoning section: in shopping, cooking, travel, shape and volume questions. The Department publishes no topic list, only a range of maths content areas2NSW Education · 13 October 2026 · Published.

PartThe Department's rule
Questions and time352NSW Education · 12 October 2026 · Published questions in 402NSW Education · 12 October 2026 · Published minutes
Options per question52NSW Education · 12 October 2026 · Published
CalculatorNo calculator2NSW Education · 12 October 2026 · Published
Working paper23NSW Education · 12 October 2026 · Published sheets, more on request
Maths neededconcepts your child already learns at school2NSW Education · 12 October 2026 · Published

Without a calculator, multiplying and dividing by ten, a hundred or a thousand has to be quick. It's moving the digits, not long multiplication.

4

What is the metric staircase?

The metric staircase puts units in order from big to small. Walking down to a smaller unit, you multiply, because one big unit breaks into many small pieces. Walking up to a bigger unit, you divide.

The metric staircase: km to m is × 1000, m to cm × 100, cm to mm × 10. Down to a smaller unit multiply, up divide.
MeasureStep down (multiply)Step up (divide)
Length1 km = 1000 m1000 m = 1 km
Length1 m = 100 cm100 cm = 1 m
Length1 cm = 10 mm10 mm = 1 cm
Mass1 kg = 1000 g1000 g = 1 kg
Capacity1 L = 1000 mL1000 mL = 1 L
Volume to capacity1 cm³ holds 1 mL1000 cm³ holds 1 L

The lesson's magic number is a thousand: it links kilometres and metres, kilograms and grams, and litres and millilitres.

5

What is the method for unit questions?

Spot the units, pick one to work in, convert, then answer in the unit asked. The lesson's single most important question is: am I going to a bigger or a smaller unit?

  1. Spot every unit

    Find each unit in the question and in the options. Mixed units are a sign to convert first.

  2. Choose one unit to work in

    Usually the smaller unit, because it avoids decimals. Grams instead of kilograms, millilitres instead of litres.

  3. Up or down the staircase

    Going to a smaller unit, multiply. Going to a bigger unit, divide. Then ask whether the new number should be bigger or smaller.

  4. Answer in the unit asked

    Read the last line again. If the question wants kilograms, convert back before choosing.

6

What does a worked example look like?

We wrote this question to show working in one unit.

Our practice question
A jug holds 2.4 L of juice. Ben pours three glasses of 350 mL each. How much juice is left in the jug, in millilitres?
StepWhat your child does
Spot the unitsLitres and millilitres, and the answer is wanted in millilitres
Choose one unitMillilitres, the smaller unit
ConvertLitres to millilitres is a step down, so multiply: 2.4 × 1000 = 2400 mL
CalculateThree glasses: 3 × 350 = 1050 mL. Left: 2400 − 1050 = 1350 mL
Check1350 mL is about 1.35 L, a bit more than half the jug. That makes sense

A child who takes 1050 from 2.4 without converting gets a number that can't be right. Converting first stops that slip.

7

Which mistakes catch most children?

The number one trap is using the wrong operation: multiplying when you should divide. The fix is to ask which way you're going on the staircase every time.

TrapWhat it looks likeThe fix
The wrong operation2500 g turned into 2,500,000 kgSmall to big means divide: 2500 ÷ 1000 = 2.5 kg
Comparing different unitsThinking 800 m is more than 1.2 kmConvert first: 1.2 km = 1200 m
Assuming each mark is oneTaking every small line on a jug as 1 mLWork out what one small mark is worth
8

Can my child try one on their own?

Here is a measuring jug question from our free maths set. Your child answers it, then checks the steps below.

Practice question · SEL-MA-TST-13

Eli is making cordial. The picture shows the water already in his measuring jug.

The recipe needs 1 L of water. How much more water must Eli add?

  1. A

    200 mL

  2. B

    300 mL

  3. C

    380 mL

  4. D

    700 mL

  5. E

    1700 mL

Show answer and explanation

Answer: B

Answer: B (300 mL)

  1. The labels go up by 200 mL, and there are 4 equal small spaces between labels. Each small space is 200 ÷ 4 = 50 mL.
  2. The water is 2 small spaces above 600 mL: 600 + 2 × 50 = 700 mL.
  3. 1 L = 1000 mL, so Eli must add 1000 − 700 = 300 mL.

Why the other options are wrong

  • A (200 mL): This counts each small space as 100 mL, so it reads the water as 800 mL.
  • C (380 mL): This counts each small space as 10 mL, so it reads the water as 620 mL.
  • D (700 mL): This is the water already in the jug, not the amount to add.
  • E (1700 mL): This adds the water in the jug to 1 L instead of finding the difference.

  1. Read the scale

    The labels go up by 200 mL, with 3 small marks between them. That makes 4 equal spaces, so each is 200 ÷ 4 = 50 mL.

  2. Read the water

    The water is 2 small spaces above 600 mL: 600 + 2 × 50 = 700 mL.

  3. Convert and subtract

    1 L = 1000 mL, so Eli adds 1000 − 700 = 300 mL. The answer is B.

9

How can my child practise unit conversions at home?

The kitchen and the car are full of units. A few quick questions a day build the habit of checking which way to go.

Read the labels

Packets list grams, bottles list millilitres. Ask what the amount is in kilograms or litres.

Cook together

Measure with a jug and scales, and read the scale's small marks before pouring.

Trip distances

Change road-sign kilometres into metres, then compare two trips in the same unit.

10

Questions parents ask about unit conversions for Selective

What is the easiest way to remember multiply or divide?

Big to small, multiply. Small to big, divide. The lesson calls it the golden rule.

Which conversions should my child know by heart?

The ones in the staircase table above, especially the thousands: kilometres and metres, kilograms and grams, litres and millilitres.

Are time conversions part of this lesson?

No. Hours and minutes don't go up in tens, so they have their own lesson, Time Calculations.

Is the full lesson free?

A free account opens one lesson, How and Why Authors Write; the other lessons need a paid plan1SelectiveReady · 13 October 2026 · Published.

Next questions

Selective Mathematical Reasoning →Lesson: Time Calculations →Lesson: Fractions, Decimals & Percentages →Lesson: 3D Shapes & Space →Free Selective practice test →

Sources and checks

  1. SelectiveReady

    SelectiveReady · Checked 13 October 2026

  2. Selective high school practice tests

    NSW Department of Education · Checked 12 October 2026 · Practice materials: personal or non-commercial use only; CC BY 4.0 does not apply (page notice). Page text: CC BY 4.0.

  3. Placement test

    NSW Department of Education · Checked 12 October 2026 · CC BY 4.0 (NSW Department of Education copyright notice); quotes credited

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