Perimeter - KS3 Maths - BBC Bitesize (2024)

• is a length measured in either metric units such as millimetres, centimetres and metres, or imperial units such as inches and miles.

• The perimeter is found by adding together the lengths of each side. It may be necessary to convert between metric units to ensure that the added lengths are measured in the same units.

• For a the perimeter can be calculated by multiplication. There are two different formulae for finding the perimeter of a rectangle. Using formulae and changing the subject of a formula are useful skills for solving perimeter problems.

The perimeter may be found by counting squares along all the edges of a shape, or by adding up all lengths of the sides of any shape or by using a formula for a specific shape.

• To find the perimeter of a or a drawn on a grid:

  1. Pick a starting (corner).
  2. Count the squares along each side, going around the whole shape back to the starting point.

• To find the perimeter of a shape where the lengths are given for each side:

  • Write down all the lengths, in the same units, and add them up.

• To find the perimeter of a shape using a formula:

  • Choose the correct formula
    1. For a square \(P\) = 4\(s\) where \(s\) is the length of one side
    2. For a rectangle either \(P\) = 2\(l\) + 2\(w\) or \(P\) = 2(\(l\) + \(w\)), where \(l\) is the length and \(w\) the width of the rectangle.
  • Substitute the values into the formula and complete the calculation.

Once the perimeter has been worked out, write the answer with the correct units.

Perimeter problems may involve working out a length or width or finding the perimeter of a different shape. A problem may be solved by:

• A numerical method

• Rearranging a formula

• Using known facts to simplify the problem

• Using a numerical method to find a length, given the total perimeter:

  1. Add up all the given side lengths.
  2. Subtract from the perimeter.

• Rearranging a formula to find a length:

  1. Substitute the given values into the formula.
  2. Use inverse operations to undo each step to find the missing length.

• To solve a perimeter problem when information is given in an unfamiliar way:

  • Use the known facts to build understanding.

A shape is made up of two or more rectangles.

The perimeter may be found by counting squares along all the edges a shape or by adding up all lengths of the sides of the shape.

• For a rectilinear compound shape drawn on a grid:

  1. Pick a starting vertex (corner).
  2. Count the squares along each side, going around the whole shape back to the starting point.

• For a shape where the lengths are given for each side:

  • Write down all the lengths, in the same units, and add them up.

• For a shape where lengths are missing:

  1. Work out the missing lengths, using edges.
  2. Write down all the lengths, in the same units, and add them up.
  3. If the lengths cannot be worked out, use known facts to find the perimeter.

Fencing companies install perimeter fences for different clients, such as around businesses, private properties, parks and gardens.

A perimeter fence is part of a security system. The fencing company will measure the lengths around the site, adding them up to find the total length of perimeter fencing that is needed.

This then affects how much they will charge for the amount of fencing their client needs.