Perimeter Of Rectangle Calculator

What this calculator does

So, you want the distance all the way around a rectangle. This tool finds it. Enter the length and the width, choose your unit, and it returns the perimeter, the total length of the four sides added together.

Two measurements and a unit, and the perimeter comes straight back.

How to use it

1. Enter the length of the rectangle.
2. Enter the width.
3. Choose the unit, then press Calculate.

What perimeter means

The perimeter of a shape is the distance around its outside edge. If you walked all the way around the boundary of a rectangle and measured how far you had gone, that distance is the perimeter. Because it is a length, it is measured in ordinary units of length such as metres or feet, not in square units, which are for area. Perimeter and area answer different questions: area is how much surface a shape covers, while perimeter is how long its border is. This tool is about that border.

Two lengths and two widths

A rectangle has four sides, but they come in two matching pairs: two long sides of equal length and two short sides of equal width. So to go all the way around, you cover the length twice and the width twice. That is why the perimeter of a rectangle is two times the length plus two times the width, which is the same as twice the sum of the length and the width. The tool adds the length and width together and doubles the result. You only need to measure two sides, because the opposite ones match.

Choosing your unit

The tool lets you pick the unit your measurements are in, from millimetres and centimetres up to metres, kilometres, and miles, as well as inches, feet, and yards. Whichever you choose, the perimeter comes back in that same unit of length, since adding up sides measured in centimetres gives a total in centimetres. The important thing is to enter both the length and the width in the same unit, so they can be added sensibly. Picking the right unit just keeps the answer at a comfortable size for whatever you are measuring, whether a postage stamp or a football pitch.

A worked example

Take a rectangle 5 units long and 3 units wide. Add the length and width to get 8, then double it to get 16, so the perimeter is 16 units. You can check it the long way: the two lengths are 5 and 5, the two widths are 3 and 3, and 5 plus 5 plus 3 plus 3 is 16. Both routes agree.

The square as a special case

A square is really just a rectangle whose length and width happen to be equal. If you put the same number into both the length and the width here, the formula still works: two times the side plus two times the side is four times the side, which is exactly the perimeter of a square. So the rectangle formula quietly contains the square formula inside it. When all four sides are the same, the dedicated perimeter of a square calculator is the quicker route, since it asks for only one measurement.

Questions people ask

What is the perimeter of a rectangle?

The total distance around its four sides, equal to two times the length plus two times the width.

Why double the sum of length and width?

Because a rectangle has two equal lengths and two equal widths, so you cover each measurement twice going around.

Does the answer use square units?

No. Perimeter is a length, so it is measured in plain units like metres or feet. Square units are for area.

Do I enter length and width in the same unit?

Yes. Both measurements must be in the same unit so they can be added, and the perimeter comes back in that unit.

What if all four sides are equal?

Then it is a square, and the perimeter is four times the side. The perimeter of a square calculator handles that case.

References

On the perimeter of a rectangle. The perimeter is the distance around the boundary, equal to twice the sum of the length and width.

1. Eric W. Weisstein, "Perimeter," from MathWorld, a Wolfram resource, on the perimeter as the length of a shape's boundary.
2. Eric W. Weisstein, "Rectangle," from MathWorld, a Wolfram resource, on the rectangle, a quadrilateral with opposite sides equal and four right angles.