Grade 7 Mathematics MEASUREMENT – Length Notes
MEASUREMENT — Length
For learners (age 12) — Kenya
What is Length?
Length is how long or short something is. We measure length to compare sizes, to build things, and to know how far apart places are.
Common Metric Units
- Kilometre (km) — used for long distances (between towns, roads).
- Metre (m) — base unit. Good for rooms, fields, height of a person.
- Centimetre (cm) — used for small objects (books, rulers).
- Millimetre (mm) — for very small lengths (thickness of a card).
How the Metric Units relate
These are powers of ten:
1 km = 1000 m
1 m = 100 cm
1 cm = 10 mm
Quick rule: move the decimal point.
- To convert m → cm: multiply by 100 (move decimal 2 places right). Example: 1.25 m = 125 cm.
- To convert cm → m: divide by 100 (move decimal 2 places left). Example: 250 cm = 2.5 m.
Visual: A simple ruler (30 cm)
The ruler above shows centimetres and smaller mm ticks. A 30 cm ruler is common in school.
Tools for measuring length
- Ruler (30 cm) — for small objects.
- Tape measure — for furniture, rooms, and clothes.
- Trundle wheel — for measuring long distances on the ground (fields, school compound).
- Metre rule — a straight 1 m rule for classrooms or carpentry.
Measuring tips (easy to follow)
- Always start measuring from the zero mark, not from the edge of the ruler which may be worn.
- Keep the ruler or tape straight along the object. For long objects, use a tape measure or ask a friend to help.
- Read to the nearest mm or nearest half cm if the length is between marks. Estimate if needed.
- Write units clearly: 45 cm (not just 45).
Worked examples
Example 1 — Convert:
Convert 3.7 m into centimetres.
Method: 1 m = 100 cm → multiply metres by 100.
3.7 × 100 = 370 cm
Answer: 3.7 m = 370 cm
Example 2 — Real-life:
A classroom is 8 m long and 6 m wide. What is its perimeter?
Perimeter of rectangle = 2 × (length + width) = 2 × (8 + 6) = 2 × 14 = 28 m
Answer: 28 m
Practice questions
- Convert 1250 m into kilometres. (Hint: 1 km = 1000 m)
- Convert 4500 mm into metres. (Hint: 1000 mm = 1 m)
- Measure: If your school desk length is 120 cm and width is 60 cm, what is its perimeter in metres?
- Estimate: Which is longer — the length of a soccer pitch (≈ 100 m) or walking from home to nearby market (2 km)? Explain in one sentence.
- True or false: 200 cm = 2 m.
Answers
- 1250 m = 1.25 km
- 4500 mm = 4.5 m
- Perimeter = 2 × (1.20 m + 0.60 m) = 2 × 1.8 = 3.6 m
- Walking 2 km is longer than a soccer pitch of about 100 m because 2 km = 2000 m.
- True. 200 cm = 2.00 m.
Practise measuring many objects at home and in school (books, door width, classroom length). Convert your measurements between mm, cm, m, and km to become confident with length.