Measurement
Perimeter: once round the outside
Find the perimeter of any straight-sided shape by adding the lengths of its sides, and work backwards to a missing side.
The perimeter of a shape is the distance once round the outside of it — the length of fence you would need, or how far you would walk if you followed the edge back to where you started. It works for any straight-sided shape at all, however lopsided: you add the sides, and that is the whole idea.
Add every side, and only once
The two mistakes worth knowing about are missing a side out and counting one twice. Pick a corner, go round the same way all the way back to it, and tick each side off as you pass it. A shape with equal sides lets you multiply instead of adding — five sides of 4 is 5 × 4 — but that is a shortcut, not a different rule.
Have a play
An L-shaped garden, with every side drawn as its own strip. Find all of them, and count how many there are.
Tap any part of the picture.
Worked example
A triangle has sides of 6 cm, 8 cm and 5 cm. What is its perimeter?
Start at one corner and add the first two sides: 6 + 8 = 14.
Two thirds of the way round, keeping count as you go.
Try it together
Now let us find the perimeter of a rectangle 12 by 7 together.
Everything here is measured in centimetres. A rectangle has two long sides and two short ones.
1.Add one long side and one short side. What is 12 + 7?
Have a go
Have a go on your own: a regular pentagon has sides of 8 cm. What is its perimeter, in centimetres? Give just the number.
Hint: A pentagon has five equal sides.
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.