Perimeter of a Rectangle Calculator

Created by Dominik Czernia, PhD candidate
Last updated: Jun 01, 2020

Use our perimeter of a rectangle calculator if you need to quickly estimate what is the perimeter of a specific rectangle. Try to enter some values or read on to learn more about rectangles. In the following text, we explain in detail how to find the perimeter of a rectangle with 10 different perimeter of a rectangle formulas.

Rectangle is a quadrilateral with four right angles (4 * 90° = 360°). Its name is derived from the Latin word rectangulus which means right (rectus) angle (angulus). The opposite sides of rectangle are parallel to each other and of equal length. Rectangle has two diagonals which intersect in the middle of the rectangle and are of equal lengths. In the picture below, you can see a typical rectangle with marked parameters:

A rectangle with five basic quantities and a perimeter of a rectangle
  • l - length,
  • w - width,
  • α - angle between diagonals,
  • r - circumcircle radius,
  • d - diagonal,

There are two other characteristic quantities which are not shown in the picture:

  • A - area,
  • P - perimeter.

You can always circumscribe a circle on the rectangle because its center is equidistant from all of its four vertices. Moreover, the center of this circle lies exactly at the intersection of two diagonals. However, you can't inscribe a circle into every rectangle. In fact, you can do it only with a square which is special case of a rectangle. Square is a quadrilateral with four right angles and all four sides of equal lengths. Check out our perimeter, area and diagonal of a square calculators if you need to solve specific problems with squares!

How to find the perimeter of a rectangle?

A perimeter is a path that surrounds any two-dimensional shape. You can think of it like a fence that is required to surround a yard of a garden. A circle is a special figure, because its perimeter is usually called the circumference.

So how to find the perimeter of a rectangle? You need to sum up the lengths of every side:

P = l + w + l + w

P = 2 * l + 2 * w

P = 2 * (l + w)

How to find the perimeter of a rectangle? - a rectangle with length and width

With this perimeter of a rectangle calculator, you can make calculations in almost any units you want. To learn more about length units, check out our length converter!

What's the perimeter of a rectangle formula?

To calculate the perimeter in the above equations, we have used two sides of a rectangle. However, in some mathematical problems, there are different quantities given. How to find the perimeter of a rectangle in these situations? Most of these problems can be solved with our perimeter of a rectangle calculator. Before we write down appropriate formulas, there are three basic equations for the area, diagonal and circumcircle radius of a rectangle that you should remember:

  1. Area of a rectangle: A = w * l,
  2. Diagonal of a rectangle d = √(l² + w²),
  3. Circumcircle radius of a rectangle r = d/2.

With the above equations we can now derive various perimeter of a rectangle formulas that are used by calculator on this site:

  1. Given length and width: P = 2*l + 2*w,
  2. Given length/width and diagonal: P = 2*l + 2*√(d² - l²) or P = 2*w + 2*√(d² - w²),
  3. Given lenght/width and area: P = 2*l + 2*A/l or P = 2*w + 2*A/w,
  4. Given length/width and angle: P = 2*l + 2*l/tan(α/2) or P = 2*w + 2*w*tan(α/2),
  5. Given length/width and circumcircle radius: P = 2*l + 2*√(4*r² - l²) or P = 2*w + 2*√(4*r² - w²),
  6. Given diagonal and area: P = √[2*d² + 2*√(d⁴ - 4*A²)] + √[2*d² - 2*√(d⁴ - 4*A²)],
  7. Given diagonal and angle: P = d * (2*sin(α/2) + 2*cos(α/2)),
  8. Given area and angle: P = 2*√[A / tan(α/2)] * (tan(α/2) + 1),
  9. Given area and circumcircle radius: P = √[8*r² + 4*√(4*r⁴-A²)] + √[8*r² - 4*√(4*r⁴-A²)],
  10. Given angle and circumcircle radius: P = 2 * r * (2*sin(α/2) + 2*cos(α/2)).

Note: The angle α between diagonals is in the front of the length like in the first figure. Also, remember that the perimeter of a rectangle calculator assumes that the length is longer than the width!

Have you ever heard about the golden rectangle? It is a special rectangle, the side lengths of which are in golden ratio. Check out our golden rectangle calculator to learn more about constructing golden rectangles!

Dominik Czernia, PhD candidate
How to find th perimeter of a rectangle - a picture
Width (a)
Length (b)
Diagonal (d)
Angle between diagonals (γ)
Circumcircle radius
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