Length and distance
- Length is how long an object is: a pencil, a table, a hallway.
- Distance is how far apart two places are: home to school, Toronto to Ottawa.
Both use the same units.
Choosing a tool
| Tool | Best for |
|---|---|
| Ruler (30 cm) | small objects |
| Metre stick | desks, doorways |
| Measuring tape | rooms, bodies, curved objects |
| Trundle wheel | playgrounds, fields |
| Odometer or map | roads and long trips |
Choosing a unit
| Unit | Use for |
|---|---|
| Millimetre (mm) | very small things: an ant, a screw |
| Centimetre (cm) | everyday objects: a book, a hand |
| Metre (m) | rooms, heights, pools |
| Kilometre (km) | distances between places |
How to measure with a ruler
- Line up the object’s end with the zero mark, not the edge of the ruler. Many rulers have a small gap before zero.
- Keep the object straight along the ruler.
- Read the mark at the other end.
- Count small marks for millimetres: each small mark is 1 mm, and 10 mm = 1 cm.
- Write the number and the unit: 8.4 cm or 84 mm.
Reading between the marks
On a centimetre ruler, the long lines are whole centimetres, the middle line is half a centimetre (5 mm), and the small lines are millimetres.
If a pencil ends 3 small marks past 12 cm, it is 12.3 cm (123 mm) long.
Measuring when you can’t start at zero
If the end of a ruler is broken, start at any whole number and subtract. An object from 4 cm to 11.5 cm is 11.5 − 4 = 7.5 cm long.
Measuring curved lines
Lay a piece of string along the curve, mark the end, then straighten the string and measure it with a ruler.
Estimating length
Use body benchmarks:
- Width of a fingernail ≈ 1 cm
- Hand span ≈ 15–20 cm
- One big step ≈ 1 m
Worked examples
Example 1: A pen starts at 0 and ends at 14.6 cm. How long is it in mm?
14.6 × 10 = 146 mm.
Example 2: A leaf goes from the 3 cm mark to the 9 cm mark. How long is it?
9 − 3 = 6 cm.
Example 3: Which unit for the distance from Vancouver to Calgary?
Kilometres (it’s about 1,000 km by road).
Common mistakes
- Starting at the end of the ruler instead of at zero.
- Forgetting the unit, or using the wrong one.
- Reading millimetre marks as centimetres.
Try it yourself
- A crayon measures 9 cm and 5 mm. Write this in mm.
- An object goes from 2 cm to 10.5 cm. How long is it?
- Which unit for the thickness of a coin?
- Convert 3.5 km to metres.
Answers: 1) 95 mm 2) 8.5 cm 3) mm 4) 3,500 m
Quick recap
- Measure length and distance with the right tool and unit.
- Always start at the zero mark.
- 10 mm = 1 cm; 100 cm = 1 m; 1,000 m = 1 km.