Grade 5 Mathematics Measurement – Length Notes
Mathematics — Measurement
Subtopic: Length (for age 10)
Learning goals
- Know what length means.
- Use common metric units: mm, cm, m, km.
- Convert between units (mm ⇄ cm ⇄ m ⇄ km).
- Measure objects with a ruler or tape measure and compare lengths.
What is length?
Length is how long something is — from one end to the other. We use units to say how long, for example: a pencil might be 15 centimetres long, a classroom 8 metres long.
Metric units of length (Kenya uses the metric system)
Millimetre (mm)
Very small. Use for tiny things (thickness of a coin).
1 cm = 10 mm
Centimetre (cm)
Good for pencils, hands, small books.
1 m = 100 cm
Metre (m)
For room length, classroom, height of a door.
1 km = 1000 m
Kilometre (km)
Long distances, e.g., distance between towns.
Quick conversion rules
- 1 m = 100 cm
- 1 cm = 10 mm
- 1 km = 1000 m
Example: Convert 2.5 m to cm → multiply by 100 → 2.5 × 100 = 250 cm.
Measuring tools
Ruler
Usually 30 cm long with marks for cm and mm. Use for small objects.
Tape measure
Good for longer objects: desks, tables, or the length of a room. It usually shows metres and centimetres.
Tape: 0 — 1 m (example)
How to measure with a ruler (step-by-step)
- Place the ruler so the 0 mark is at one end of the object.
- Read the mark at the other end — that number is the length.
- If the end falls between marks, read the nearest mm or estimate to the nearest mm/cm.
Tip: Keep the ruler straight along the object. If the object is longer than your ruler, mark a point and move the ruler, then add the lengths.
Comparing and ordering lengths
To compare: measure both objects in the same unit (e.g., both in cm). The larger number means the longer object.
Example: Book A = 21 cm, Book B = 18 cm → Book A is longer.
Perimeter (short mention)
Perimeter is the total length around a shape. To find the perimeter of a rectangle: add all sides or use 2 × (length + width).
Example: rectangle 6 m by 3 m → perimeter = 2 × (6 + 3) = 18 m.
Practice questions
- Convert 450 mm to cm.
- A pencil is 17 cm long. What is its length in mm?
- How many metres are in 3.4 km?
- Measure the width of your notebook with a ruler and write the length (teacher asks class).
- Find the perimeter of a rectangle with length 5 m and width 2 m.
Answers:
- 450 mm = 45 cm (divide by 10).
- 17 cm = 170 mm (multiply by 10).
- 3.4 km = 3,400 m (multiply by 1000).
- (Student activity — measure in cm or mm.)
- Perimeter = 2 × (5 + 2) = 14 m.
Remember: Always use the same unit when comparing lengths. For most school work use metres (m) and centimetres (cm).