Grade 3 Mathematics Measurements – Length Notes

1. What is length?

Length tells us how long something is. It can be the length of a pencil, a desk or a classroom door. We measure length using the metre system (metric units) which is used in Kenya.

2. Common units of length

• Millimetre (mm) — very small unit. 1 cm = 10 mm.
• Centimetre (cm) — good for pencils, books. 1 m = 100 cm.
• Metre (m) — used for rooms, doors, table length.

Remember: 1 m = 100 cm and 1 cm = 10 mm.

3. Tools to measure length

- Ruler (usually 30 cm) for small things.
- Tape measure or metre rule for longer items (doors, desks).
- Use the correct side of the ruler (cm side) when measuring in cm or m.

4. How to measure with a ruler (easy steps)

1. Place the ruler so the 0 cm mark is at one end of the object.
2. Keep the ruler straight along the object.
3. Read the number at the other end — this is the length in centimetres (cm).
4. If the object ends between cm marks, read the nearest smaller cm and count the extra mm.

5. Visual: a 30 cm ruler

Tip: A school ruler usually shows cm and mm. Count the small lines to read mm.

6. Examples

• Pencil length = 14 cm (write 14 cm).
• School book height = 25 cm (write 25 cm).
• Classroom door height ≈ 2 m (write 2 m = 200 cm).

Convert 2 m to cm: 2 × 100 = 200 cm.

7. Activities to try at school or at home

1. Use a ruler to measure your pencil, eraser and exercise book in cm. Write the lengths.
2. Order these items from shortest to longest.
3. Measure the board or a table using a tape measure. Say the length in metres and in centimetres.

8. Practice questions

1. How many centimetres in 1 metre?
2. Write the length 7 cm in millimetres.
3. If a book is 23 cm and a pencil is 12 cm, which is longer and by how many cm?
4. Measure a classroom desk and say its length in metres and centimetres.
5. Convert 150 cm into metres.