How to find perimeter

Perimeter is the distance around a shape. Add the lengths of all the sides. For a rectangle there is a shortcut: 2 × (length + width). A 6 by 4 rectangle has a perimeter of 20. This page covers perimeter of any polygon, missing sides, the rectangle builder, and why shapes with the same perimeter can have different areas.

Perimeter: the big idea

Imagine walking all the way round the edge of a shape, or putting a fence around a garden. The total distance is the perimeter. It is a length, so it is measured in plain units: cm, m, in, ft.

How to find perimeter, step by step

  1. Find every side length. Mark each side as you count it so none is missed.
  2. Add them all.
  3. Write the unit (cm, m, in…).

Rectangle shortcut: opposite sides are equal, so P = 2 × (length + width). Square: P = 4 × side.

Perimeter and area together

Try 6 × 4 and 8 × 2. Same perimeter (20), different area.

Finding a missing side

A rectangle has a perimeter of 30 cm and a length of 9 cm. Two lengths make 18, so the two widths make 12, and each width is 6 cm (3.MD.D.8). For L-shapes, use the opposite sides: the missing horizontal pieces add up to the long horizontal side.

Same perimeter, different area

A 6 × 4 rectangle and an 8 × 2 rectangle both have a perimeter of 20, but their areas are 24 and 16. The closer a rectangle is to a square, the more area it holds for the same fence. This is a favorite grade 3 problem.

Common mistakes

Multiplied the sidesFound area instead.Fix: perimeter adds every side.
Counted only two sides of a rectangleAdded length + width once.Fix: a rectangle has four sides: 2 × (length + width).
Missed a side on an L-shapeSome sides aren’t labeled.Fix: work out the missing sides first, then mark each one as you add.
Square units for perimeterWrote 20 cm².Fix: perimeter is a length: 20 cm.

How to teach it at home

Walk the edge

Pace out the perimeter of a room or the garden.

String shapes

Use a 24-inch string to make different rectangles. Which holds the most?

Picture frames

Measure a photo and work out how much frame it needs.

By grade

Worksheets and practice

Perimeter worksheet, page 1

Perimeter worksheet: printable PDF with an answer key. · Make a fresh test

Perimeter and area chart · Geometry

Last reviewed

Questions parents ask

How do you find perimeter?

Add the lengths of all the sides. For a rectangle, 2 × (length + width).

What is the perimeter of a square?

4 × the side length. A square with 5 cm sides has a perimeter of 20 cm.

What is the difference between perimeter and area?

Perimeter is the distance around a shape; area is the space inside it.

How do you find a missing side from the perimeter?

Subtract the known sides from the perimeter. For a rectangle, halve the perimeter and subtract the length to get the width.

What grade learns perimeter?

Grade 3 (3.MD.D.8).