Imagine an ant standing at the corner of a rectangular garden. It walks along every edge until it's back where it started. The total distance it walks has a special name in maths: the perimeter.
By the end of today, you'll be able to work out that distance for almost any flat shape — even tricky ones made by joining shapes together!
Press start and watch the ant trace every edge of the garden. When it's back at the start, that total distance is the perimeter.
Before pressing start — roughly how far do you think the ant will walk in total, if the garden is 8 cm long and 5 cm wide?
Drag each real-life situation into the box where it belongs — is it about the edge/outline of something, or the flat surface? This helps us see what you already know about "going around" a shape.
🖱️ Drag a card onto the correct box (or tap a card, then tap the box, on touch devices).
Perimeter is the total distance all the way around the outside edge of a flat (2D) shape. It's measured in units of length, like cm, m, inches or feet — never in "squared" units.
The total distance around the outside edge of a 2D shape. You find it by adding up the lengths of every side.
How much fencing you need to go around a garden.
How much wooden frame you need around a picture.
How far you run doing one lap of a running track.
A shape has 4 sides measuring 3 cm, 5 cm, 3 cm and 5 cm. What is its perimeter?
A rectangle has 2 pairs of equal, opposite sides — a length and a width. A square is a special rectangle where all 4 sides are equal.
A square field has a side length of 12 m. What is its perimeter?
A triangle has 3 sides. There's no shortcut formula with multiplying for a general triangle — you simply add all 3 sides together. For a special equilateral triangle (all sides equal), you can multiply one side by 3.
This right-angled triangle has sides 6 cm, 8 cm and 10 cm. Watch the total build up as the ant walks each side.
All 3 sides equal
2 sides equal
No sides equal
An equilateral triangle has a side of 7 cm. What is its perimeter?
A regular polygon is a shape where every side is the exact same length (like a regular pentagon or hexagon). Instead of adding the same number again and again, we can multiply!
A regular octagon (8 sides) has each side 3.5 cm. What is its perimeter?
A parallelogram has 2 pairs of equal, opposite sides — just like a rectangle — but it's "pushed over" at a slant. This means the same formula idea works, even though the shape looks tilted.
A parallelogram has sides of 11 cm and 6 cm. What is its perimeter?
Some shapes are made by joining two or more plane shapes together — like an L-shape made from two rectangles. Sometimes not every side length is given, so you must work out the missing sides first, then add everything on the outside edge.
Why can't you find the perimeter of the L-shape by just adding the 4 given numbers (10, 8, 6, 4)?
Sometimes you're given the perimeter and asked to find a missing side instead! The trick is to rearrange the formula, step by step — just like solving a puzzle.
An equilateral triangle has a perimeter of 33 cm. What is the length of one side?
Drag each shape to the formula it matches — or tap a shape, then tap the formula it matches.
12 questions, getting a little harder as you go. Good luck!
Try explaining to a family member: how would you find the perimeter of your own bedroom floor? Use the words perimeter, length, width, and add in your answer.
Fantastic work finding the perimeter of rectangles, triangles, polygons, parallelograms, composite shapes, and missing sides. Jump back to any section any time using the map below.