Label the sides the diagram left blank: perimeter
The Short Answer
Perimeter is the distance all the way round the edge of a shape, measured in ordinary units such as centimetres. On an 11+ paper the arithmetic is trivial and the question is really about the sides the diagram has not labelled — on any shape made of right angles, the short sides across add up to the long side opposite them.
What is a perimeter question?
The distance all the way round the edge of a shape. Your child has known that for years.
On a real 11+ paper the adding is not the question. The question is the sides the diagram has not labelled — and working those out is where the marks are.
Around or inside?
Before anything else, underline the word in the question.
Perimeter is around the edge, so the answer is in centimetres or metres.
Area is the space inside, so the answer is in square centimetres or square metres.
The units are a check as well as an instruction: an answer in cm² to a perimeter question is wrong before the arithmetic is looked at.
The rule for the missing sides
This works on any shape made of right angles, which is nearly all of them at this level.
The short horizontal sides across the shape add up to the long horizontal side opposite them. And the same is true going up and down.
So a shape that is 15 cm along the bottom with a top edge of 9 cm has a missing horizontal of 15 − 9 = 6 cm.
Can you show me a worked example?
Where do pupils lose marks?
Missing a side, by adding in the order the eye finds them.
Giving the answer in square units, which is the area habit arriving in the wrong question.
Not labelling the blanks, and then guessing at them later.
Being given the area and treating it as a side length.
Try it yourself
Label every unlabelled side before you add anything. Then travel round the shape once, in one direction.
- A rectangle is 12 cm by 7 cm. What is its perimeter?
- An L-shape is 15 cm along the bottom and 10 cm up the left side. The top edge is 9 cm. What is the missing horizontal side?
- If that L-shape's right-hand side is 4 cm, what is the remaining vertical side?
- What is the perimeter of that L-shape?
- Why is going round in one direction better than adding the sides in any order?
- 38 cm. Two lots of 12 plus two lots of 7. Note the unit — centimetres, not square centimetres, because nothing here is about the space inside.
- 6 cm. The two horizontal sides across the top must add up to the 15 cm along the bottom, so the missing one is 15 − 9.
- 6 cm. The same rule going up and down: the verticals on the right must add to the 10 cm on the left, so 10 − 4 = 6.
- 50 cm. Travelling round: 15 + 10 + 9 + 4 + 6 + 6. Every side counted once, in order.
- Because it is the only way to be certain nothing is counted twice and nothing is missed. Picking sides off a diagram in whatever order they catch the eye is how a six-sided shape ends up with five sides added.
How much practice does this type need at home?
The arithmetic needs none. The labelling needs a little, and it can be done with a pencil and a scrap of paper.
Draw an L-shape, label three sides, and ask your child for the other three. Then swap — let them draw one for you. Drawing one that works requires understanding the rule from the other direction, which is a quicker way to learn it than doing another six questions.
Free: a Maths workbook
Questions across the Maths question types, with the method printed at the top and full answers at the back. One workbook for the whole strand, not a sheet per question type.
Frequently Asked Questions
How does my child work out a side the diagram has not labelled?
On any shape made of right angles there is one rule and it works every time: the short horizontal sides across the shape add up to the long horizontal side opposite them. The same is true going up and down. So a shape 15 cm along the bottom with a 9 cm top edge has a missing horizontal of 6 cm. Most of the time on these questions should go here, because this is the actual question.
What is the difference between perimeter and area, in a sentence a child will keep?
Perimeter is the fence; area is the grass. Or in the words we use in lessons: around or inside? Underline the word in the question before doing anything, because the two calculations are quite different and the units give the answer away — centimetres for perimeter, square centimetres for area.
Why does my child sometimes miss a side?
Because they add the sides in the order their eye finds them rather than travelling round the shape. On a six-sided L, it is very easy to add five. Put a finger on one corner, go round in one direction, and say each length aloud as you pass it. It feels slow and it takes about four seconds.
Can a question give the area and ask for the perimeter?
Yes, and it is a common way to make the question harder without making the shape harder. If a rectangle has an area of 48 cm² and one side of 6 cm, the other side is 8 cm, and only then can the perimeter be found. Read carefully for which one you have been given, because the numbers alone will not tell you.
Sources
- GL Assessment — 11+ familiarisation materials and question type descriptions Published 2025.
- Clever Clogs Academy — our own Year 4 and Year 5 lesson records, Winslow Published September 2025 to July 2026.
We are not affiliated with, endorsed by or connected to GL Assessment or Buckinghamshire Council. Test arrangements change; always check the council's own pages for the current year's dates.
We Teach This Method Every Week in Winslow
Groups of no more than six pupils with two qualified teachers, in person or online. Come and see a session before you decide.