Rectangle Calculator

Area, Perimeter, and Diagonal

A rectangle is described by just two numbers - its length and width - and three useful measurements come straight out of them:

• Area is how many unit squares fit inside. Lay them in rows and columns and you get $\text{length} \times \text{width}$.
• Perimeter is the distance all the way around the edge: two lengths plus two widths.
• Diagonal is the straight line from one corner to the opposite one. It is the hypotenuse of the right triangle made by the length and width, so it comes from the Pythagorean theorem.

The figure redraws the rectangle to match your numbers, so a long thin rectangle actually looks long and thin.

Rectangle Formulas

$$\begin{aligned} \text{area} &= \text{length} \times \text{width} \\ \text{perimeter} &= 2 \times (\text{length} + \text{width}) \\ \text{diagonal} &= \sqrt{\text{length}^2 + \text{width}^2} \end{aligned}$$

The diagonal comes from the Pythagorean theorem, since it is the hypotenuse of the triangle formed by the length and width.

How to Use It

1. Enter the length and the width.
2. Read the area, perimeter, and diagonal.

Worked Examples

Length	Width	Area	Perimeter	Diagonal
6	4	24	20	7.21
10	5	50	30	11.18
8	8	64	32	11.31

FAQ

How do you find the area of a rectangle?

Multiply the length by the width. For 6 by 4: $6 \times 4 = 24$.

How do you find the perimeter?

Add the length and width and double it: $2 \times (6 + 4) = 20$.

How do you find the diagonal?

Use the Pythagorean theorem on the length and width: $\sqrt{6^2 + 4^2} \approx 7.21$.

What if length and width are equal?

Then it is a square — for example, 8 by 8 gives an area of 64 and a perimeter of 32.