Last reviewed: 11 November 2026
Updated November 2026. These Grade 7 length notes show how to convert between centimetres, decimetres, metres, decametres and hectometres, how to add or subtract lengths, how to find the perimeter of a plane figure, and how to find the circumference of a circle. Length is the SI quantity whose unit is the metre, as set out on the NIST page on SI units of length. A football field marked 100 m is the same length as 10 000 cm or 1 hectometre.
The examples are original practice for Kenyan junior school learners, not an official paper. They follow the Grade 7 Mathematics measurements sub-strand on length in the KICD Grade Seven curriculum designs. Area of the same shapes is a separate sub-strand, in the Grade 7 area notes. For Grade 7 topics overall, use the Grade 7 mathematics revision guide and the Grade 7 notes hub.
On this page
Converting and Combining Units of Length
Each step to the next named unit on this list is a factor of 10:
| Unit | Symbol | In metres |
|---|---|---|
| centimetre | cm | 0.01 m |
| decimetre | dm | 0.1 m |
| metre | m | 1 m |
| decametre | Dm | 10 m |
| hectometre | Hm | 100 m |
So 1 m = 10 dm = 100 cm = 0.1 Dm = 0.01 Hm. To convert to a smaller unit, multiply; to convert to a larger unit, divide. Before you add or subtract, write every length in the same unit.
Worked example
Convert 3.5 m into the other units on the list: 3.5 m = 35 dm = 350 cm = 0.35 Dm = 0.035 Hm.
Add 2.4 m and 85 cm. Change 85 cm to 0.85 m, then 2.4 + 0.85 = 3.25 m. In centimetres the same sum is 240 cm + 85 cm = 325 cm.
Perimeter of Plane Figures and Circumference of Circles
Perimeter is the distance around a plane figure. For a rectangle it is 2(l + w). For a square it is 4s. For any polygon, add every side. Combined shapes, such as an L-shaped flower bed, are handled by walking around the outer edge and not adding internal cuts. Circumference is the perimeter of a circle: C = πd or C = 2πr, where d = 2r. Measuring circular objects and comparing C with d is how the design introduces π. OpenStax Prealgebra’s section on circles uses the same two formulas.
Worked example: perimeter
A rectangular classroom is 12 m long and 7 m wide. Perimeter = 2(12 + 7) = 2 × 19 = 38 m.
An L-shaped path is made from a 10 m by 8 m rectangle with a 4 m by 3 m rectangle removed from one corner. The outer walk is not 2(10 + 8) minus something guessed: sketch the shape and add the six outer sides. If the missing corner sits at the 10 m by 8 m corner, the outer sides are 10, 8, 6, 3, 4 and 5, which add to 36 m. (The 6 m side is 10 − 4 and the 5 m side is 8 − 3.)
Worked example: circumference
A bicycle wheel has diameter 14 cm. Taking π = 22/7, circumference = (22/7) × 14 = 44 cm. If you are given the radius 7 cm instead, C = 2 × (22/7) × 7 = 44 cm, the same result.
Original Practice Questions With Worked Answers
- Convert 250 cm to metres and to decametres. 2.5 m = 0.25 Dm.
- Subtract 45 cm from 3 m. 3 m − 0.45 m = 2.55 m, or 300 cm − 45 cm = 255 cm.
- Find the perimeter of a square of side 9.5 cm. 4 × 9.5 = 38 cm.
- A circle has radius 21 cm. Taking π = 22/7, find its circumference. 2 × (22/7) × 21 = 132 cm.
- A running track is a circle of diameter 70 m. How far is one lap? C = (22/7) × 70 = 220 m.
- A learner adds 2.4 m + 85 cm and writes 87.4. What went wrong? The units were not aligned. 85 cm is 0.85 m, not 85 m.
Cover the answers and try each question. The metric unit converter is a useful check on conversions. For timed practice use the Grade 7 CBC Challenge. These notes are free; full revision access is available with the KSh 20 Day Pass or the KSh 500 Monthly Pass.
Frequently Asked Questions
What length units does Grade 7 use?
The sub-strand uses centimetres, decimetres, metres, decametres and hectometres, with each step a factor of 10. 1 m = 100 cm = 10 dm = 0.1 Dm = 0.01 Hm.
How do you find the circumference of a circle?
Use C = πd or C = 2πr. A circle of diameter 14 cm has circumference 44 cm when π = 22/7.
What is the difference between perimeter and circumference?
Perimeter is the distance around any plane figure. Circumference is the special name for the perimeter of a circle.
Why must units match before you add lengths?
2.4 m and 85 cm are the same kind of quantity written in different units. Adding 2.4 to 85 as if they were alike gives a nonsense answer. Convert first, then add.