What Is Perimeter?
Mathematics · Measurement · Grades 3–5
Meaning at a glance
Perimeter is the distance all the way around the outside of a shape.
Also searched as: perimeter of a rectangle, distance around.
Understand it simply
To find the perimeter, add the lengths of all the sides. It is a length, so it is measured in ordinary units such as centimetres (cm) or metres (m).
For a rectangle, opposite sides are equal, so you can add length + width and double it: 2 × (length + width).
See it work
A rectangle is 5 units long and 3 units wide. Walk around the edge: 5 + 3 + 5 + 3 = 16 units.
Avoid this confusion
Do not forget a side. A rectangle has four sides, so add all four. Also, perimeter is not area. A farmer needs the perimeter to buy fencing, and the area to know how much grass is inside.
Try it yourself
Work these out first, then check your answers below.
A rectangle is 8 cm long and 2 cm wide. What is its perimeter?
A triangle has sides of 5 cm, 6 cm and 7 cm. What is its perimeter?
A square garden has sides of 4 m. How much fencing goes all the way round?
Check your understanding
Show the answer to question 1
Answer: 20 cm
8 + 2 + 8 + 2 = 20 cm.
Show the answer to question 2
Answer: 18 cm
Add all three sides: 5 + 6 + 7 = 18 cm.
Show the answer to question 3
Answer: 16 m
A square has four equal sides: 4 + 4 + 4 + 4 = 16 m.
Understand more with Courseversity
You now know what perimeter means. Keep going with a full lesson, then explore the ideas that connect to it.
Learn
Learn this in a full lesson.
- Perimeter (Grade 3)
- Measurement word problems (Grade 3)
- Triangles (Grade 3)
Practise
Practise with Learning Lab questions for the lesson Perimeter (Grade 3), with an explanation after each answer.
Explore
Ideas that connect to this word.
- Area: the space inside a shape, which people often confuse with perimeter.
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Sources and review
- Illustrative Mathematics: Grade 3 geometric measurement (area and perimeter) – A published reference for how area and perimeter are commonly taught. Courseversity does not claim alignment with it.
Written by: Courseversity editorial team (AI-assisted draft).
Review status: checked against the sources above. Educator review is pending, so please tell us if anything looks wrong.
Last updated: 27 October 2026.