Perimeter of a Rectangle Calculator

Perimeter of a rectangle formula

A rectangle has two pairs of equal sides. Walking once around the edge, you cover the width twice and the length twice:

P = 2 × (w + l) = 2w + 2lw is the width, l is the length, both in the same unit.

The formula can be turned around when one measurement is missing:

• Missing length from the perimeter: l = P ÷ 2 − w.
• Missing length from the area A: l = A ÷ w, after which the formula above applies.
• Diagonal, if you need it: d = √(w² + l²). A 3 by 4 rectangle has a diagonal of 5.

How to use the calculator

1. Enter the Width and the Length. They start at 3 and 4, and either may be swapped without changing the result.
2. Pick the Metric (the unit) that both measurements are in: mm, cm, dm, meter, km, in, ft, yd or mile.
3. Press Calculate to read the Rectangle perimeter, given in that same unit.

Both fields must be filled in and cannot be negative. The answer is rounded to six decimals, with trailing zeros removed in pairs, so 14 prints as 14 and 19.5 prints as 19.50. If your two measurements are in different units, convert one of them first or use the perimeter calculator, which has a unit select for each side.

Example perimeter of a rectangle problems

Example 1: fencing a garden. A garden is 12.5 m wide and 8 m long. P = 2 × (12.5 + 8) = 2 × 20.5 = 41 m. If fencing costs 18.50 per metre, the fence comes to 41 × 18.50 = 758.50.

Example 2: a picture frame. The frame around a photo is 40 cm by 30 cm. The moulding must run 2 × (40 + 30) = 140 cm, plus a little extra for the mitred corners.

Example 3: a swimming pool. A lap pool measures 25 m by 10 m, so the pool edge is 2 × (25 + 10) = 70 m long. That is the length of coping stones or edge tile you would need.

Example 4: working backwards. A rectangular plot has a perimeter of 50 m and a width of 8 m. Half the perimeter is 25, so the length is 25 − 8 = 17 m. Check: 2 × (8 + 17) = 50.

Same perimeter, different areas

Perimeter alone does not fix the size of a rectangle. All the rectangles in the table below have a perimeter of 24, yet their areas range from 11 to 36.

When width equals length the rectangle is a square, and the formula simplifies to P = 4a. The perimeter of a square calculator uses exactly that, and the area of a rectangle calculator gives the matching area.

Where the perimeter of a rectangle is used

Most everyday rectangles are measured by their edge before anything else: the fence around a yard, the baseboards in a bedroom, the moulding on a mirror, the border strip on a rug, or the tape needed to mark out a sports pitch. In each case the perimeter tells you how much linear material to buy. Add a margin of 5 to 10 percent for cuts and corners, and round up to the next full length the shop sells.

Common mistakes

• Adding the width and length only once. That gives the semi-perimeter, half of the real distance around.
• Multiplying width by length. That is the area, measured in square units, not the perimeter.
• Mixing units, such as a width in metres with a length in centimetres. Convert first, or the result is meaningless.

For shapes other than rectangles, such as triangles, circles and trapezoids, see the perimeter calculator or the circumference of a circle calculator.