how to find perimeter of a rectangle

Rectangle Shape

Rectangle Diagram with l = length and w = width
a = length side a
b = length side b
p = q = diagonals
P = perimeter
A = area
√ = square root

Calculator Use

Use this calculator if you know 2 values for the rectangle, including 1 side length, along with area, perimeter or diagonals and you can calculate the other 3 rectangle variables. A square calculator is a special case of the rectangle where the lengths of a and b are equal.

Units: Note that units of length are shown for convenience. They do not affect the calculations. The units are in place to give an indication of the order of the calculated results such as ft, ft² or ft³. Any other base unit can be substituted.

Rectangle Formulas

Area of a rectangle:

A = ab

Perimeter of a rectangle:

P = 2a + 2b

Polygon diagonals of a rectangle:

p = q = √(a² + b²)

Rectangle Calculations

Calculate A, P, p | Given a, b
Given sides lengths a and b calculate area, perimeter and diagonals
- A = ab
- P = 2a + 2b
- p = q = √(a² + b²)
Calculate P, p, b | Given A, a
Given area and side length a calculate perimeter, diagonals and side b
- b = A / a
- P = 2a + 2b
- p = q = √(a² + b²)
Calculate P, p, a | Given A, b
Given area and side length b calculate perimeter, diagonals and side a
- a = A / b
- P = 2a + 2b
- p = q = √(a² + b²)
Calculate A, p, b | Given P, a
Given perimeter and side length a calculate area, diagonals and side b
- b = (P - 2a) / 2
- A = ab
- p = q = √(a² + b²)
Calculate A, p, a | Given P, b
Given perimeter and side length b calculate area, diagonals and side a
- a = (P - 2b) / 2
- A = ab
- p = q = √(a² + b²)
Calculate A, P, b | Given p, a
Given diagonals and side length a calculate area, perimeter and side b
- b = √(p² - a²)
- A = ab
- P = 2a + 2b
Calculate A, P, a | Given p, b
Given diagonals and side length b calculate area, perimeter and side a
- a = √(p² - b²)
- A = ab
- P = 2a + 2b