Concept 4 of 8
Reading Scales & Rulers
Today's big idea
Work out what one small mark is worth before you read anything off a scale.
Between two numbered marks is where most reading mistakes are made.
How it works
👀 What's going on?
A ruler is covered in marks. A kitchen scale has a needle. A jug has lines up its side. Every one of them is useless if you read it in a hurry.
💡 Here's the trick
Look at the ruler above. The pencil tip does not land on a numbered mark at all. It stops between 6 and 7, halfway across that gap. Half of 1 cm is 5 mm, so the pencil measures 6 cm 5 mm. Reading a scale means naming the last whole mark, then measuring how far past it you have gone.
🪄 How to do it
- 1.Find where the object stops on the ruler above.
- 2.Read the last numbered mark before that point.
- 3.Count the small marks past it, and add them on.
✍️ Try it
- 1.A crayon ends 3 small marks past the 4 cm line, and each small mark is 1 mm. How long is it?
- 2.A jug is marked every 100 mL. The juice sits 7 marks above the 200 mL line. How much juice is in it?
The crayon is 4 cm plus 3 mm, which is 4 cm 3 mm, or 43 mm. The jug is harder: 7 marks of 100 mL make 700 mL, and 200 + 700 = 900 mL.
⚠️ Watch out
You might be thinking every small mark on a ruler is worth 1 mm. Look at how many pieces sit inside one centimetre before you decide. The ruler above splits each centimetre into 5 pieces, so one small mark there is 2 mm.
🌟 Why this matters
You read scales to bake a cake, to check a fever, and to cut a shelf that has to fit. A shelf 3 mm too long simply will not go in. Slow reading is what gets the right number the first time, and saves cutting the wood twice.
Exam tip: Work out what one small mark is worth before you read anything off a diagram.
Watch me solve one
Quanta solves it
“Watch how I think through one of these — then you try.”
THE PROBLEM
“A ribbon ends 2 small marks past the 8 cm line, and each small mark is 1 mm. How long is it?”
- 1
Find the last BIG mark
The nearest whole centimetre before the end is 8 cm. 🔍
- 2
Count the small marks
2 small marks past it, each worth 1 mm: 2 × 1 = 2 mm.
- 3
Join the two parts
8 cm and 2 mm together.
- 4
Answer ✓
The ribbon is 8 cm 2 mm (= 82 mm)
Now you try one — same steps, different story 👇
See it in action
⚡ Time to play!
Slide the pointer to 7 cm on the scale.
READ SCALE · ROUND 1 OF 3
0
Quick check
✅ VERIFICATION
A scale runs from 0 kg to 1 kg and is split into 10 equal spaces. What is one space worth?
⚡ Finish the activity above first to answer.
Keep going!
Up next: Measurement Word Problems
Same units, then add, subtract, or compare.