# Area of a Rectangle Calculator

With the rectangle area calculator, **you can find the area (and almost any parameter) of a rectangle**.

This rectangle area calculator doesn't just calculate the area. It can also:

- Find the dimensions of any rectangle given its area or perimeter;
- Calculate the perimeter of any rectangle; and
- Find the diagonal of any rectangle.

This is a complete rectangle calculator! Read the short article below to know more about rectangles and how to calculate the area of a rectangle.

## What is a rectangle?

A rectangle is a **four-sided irregular polygon with four right internal angles**. Every rectangle's side is equal in length to the one directly in front. Also, **its adjacent sides must be different** (otherwise, it would be a square).

This simple definition encloses some mathematical relations between a rectangle's parameters. Let's look at some of its formulas.

🙋 You can check the regular polygon calculator 📐 to learn more about this definition.

## How to calculate the area of a rectangle

Finding the area of a rectangle is really simple. All you need to do is **multiply** its length and width dimensions together:

where:

- $a$ is the rectangle's length;
- $b$ is the rectangle's width; and
- $A$ is the resulting area of the rectangle.

## How to calculate the perimeter of a rectangle

To calculate the perimeter of a rectangle, we need to add the length of each of its sides.

Alternatively, since we know that **opposite sides are equal in length**, all we need is the length of two of them.

Therefore, from its length and width, we get:

where:

- $a$ is the rectangle's length;
- $b$ is the rectangle's width; and
- $P$ is the rectangle's perimeter.

## How to find the diagonal of a rectangle

Finding the diagonal of a rectangle is the same as using the Pythagorean theorem.

Using the diagonal as the hypotenuse and the rectangle's width and length as a triangle's sides:

where again:

- $a$ is the rectangle's length;
- $b$ is the rectangle's width; and
- $d$ is the rectangle's diagonal.

## How to find the dimensions of a rectangle

Finally, let's say you want to find the rectangle's dimensions given its area or perimeter.

We can rearrange the equations to get different expressions for each dimension.

**Width**:

- $b = A/a$;
- $b = P/2 - a$; and
- $b= \sqrt{d^{2} - a^{2}}$

**Length**:

- $a = A/b$
- $a = P/2 - b$;
- $a= \sqrt{d^{2} - b^{2}}$

where:

- $a$ is the rectangle's length;
- $b$ is the rectangle's width;
- $d$ is the rectangle's diagonal;
- $P$ is the rectangle's perimeter; and
- $A$ is the rectangle's area.