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Calculate the perimeter with formulas and working

Find the length around a shape from its measurements. You can calculate the perimeter of rectangles, squares, triangles, regular polygons and circles.

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One side of the rectangle.

The other side of the rectangle.

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Worked example
  • Shape: Rectangle
  • Length: 12 m
  • Width: 10 m

Result

Perimeter

44 m

Working

  1. P = 2 × (l + b)
    2 × (12 m + 10 m) = 44 m

Perimeter is the length along the edge

The perimeter is the total distance around the edge of a shape. You can use it to calculate the length of an edge trim, mark out an area or check a geometry problem. The result is a length and is therefore shown in, for example, m or cm.

The measurements depend on the shape. A rectangle needs length and width, while a triangle needs all three sides. A square has four equal sides, so one side length is enough. Choose the shape that fits the task before you enter your measurements.

Perimeter formulas

For a rectangle, P = 2 × (l + b) is used, and for a square, P = 4 × a. The sides of a triangle are added with P = a + b + c.

A regular polygon has equal sides and equal angles. Its perimeter is P = n × a, where n is the number of sides and a is the side length. For a circle, C = 2 × π × r is used, which is the same as π × d. The radius runs from the center to the edge, while the diameter runs across through the center.

Example with a rectangle

A rectangle is 12 m long and 10 m wide. First the two measurements are added: 12 m + 10 m = 22 m. Then the sum is multiplied by two: 2 × 22 m = 44 m. The full calculation is 2 × (12 m + 10 m) = 44 m.