What is perimeter?

Part of MathsLength and distanceYear 3Year 4Year 4

What is perimeter?

Omi, looking thoughtfully at a whiteboard with a rectangle on it. An arrow is shown around the perimeter of the rectangle.

The perimeter is the distance around the outside of a 2D shape.

The distance can be measured in units like millimetres, centimetres, metres and kilometres.

To work out the perimeter, add up the lengths of all the sides.

Omi, looking thoughtfully at a whiteboard with a rectangle on it. An arrow is shown around the perimeter of the rectangle.
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Activity: What is the perimeter?

Complete this interactive activity to understand how to calculate the perimeter of 2D shapes, then put your knowledge to the test.

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Calculating the perimeter of a 2D shape

Omi has measured each side of this window.

Let's help her calculate the perimeter.

Window frame with each edge measuring 120cm, 100cm, 120cm and 100cm. Outside is a sunny day with a tree and fields. A cat lies sleeping under the tree and a robin is perched on the window sill.

To calculate the perimeter of a shape, like this window, you can add the length of all the sides together.

100cm + 120cm + 100cm +120cm = 440cm

Or, you can multiply the length and width by 2. You can also think about this as doubling.

100cm × 2 = 200cm

120cm × 2 = 240cm

You then add both answers together.

200cm + 240cm = 440cm

So the perimeter of the window is 440cm.

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Example 1

Rectangle with long edges measuring 12m each and short edges measuring 4m each.

What is the perimeter of this rectangle?

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Example 2

A square. The bottom edge has 17cm under it.

What is the perimeter of this square?

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Example 3

A rectangle with the measurement 35cm on the long edge and 14cm on the short edge.

What is the perimeter of this rectangle?

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In this game, use the times tables and more maths skills to defeat monsters and reclaim the Kingdom.

Play Guardians: Defenders of Mathematica to get ready for SATs.
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