Rectangle Calculator

Enter the length and width to find a rectangle's area, perimeter and diagonal.

Reviewed by the WorldCalcs team · Methodology · Last reviewed: June 2026

Length (l)Width (w)

Area

A = l × w

Perimeter

P = 2(l + w)

Diagonal

9.433981132

d = √(l² + w²)

What is a rectangle?

A rectangle is a four-sided shape with four right angles, where opposite sides are equal in length. The two key measurements are the length and the width. From these you can work out the area (the space it covers), the perimeter (the distance around it) and the diagonal (the straight line from one corner to the opposite corner).

The rectangle formulas

With length l and width w: the area is A = l × w, the perimeter is P = 2(l + w), and the diagonal is d = √(l² + w²). The diagonal comes from the Pythagorean theorem, because it splits the rectangle into two right triangles.

Example

A rectangle 8 long and 5 wide has an area of 8 × 5 = 40, a perimeter of 2 × (8 + 5) = 26, and a diagonal of √(8² + 5²) = √89 ≈ 9.434.

All calculations happen in your browser. Nothing is sent, stored, or tracked.

Results are estimates and may contain errors — for general information only, not professional advice. Always verify before relying on them. Disclaimer

How to use

Type positive values for the length and width. The calculator returns the area (length × width), perimeter (2 × (length + width)) and diagonal (√(length² + width²)) instantly.

Values are rounded to about 6 significant digits.

Frequently asked questions

How do you find the area of a rectangle?+

Multiply the length by the width: A = l × w. A rectangle 8 by 5 has an area of 40.

How do you find the perimeter of a rectangle?+

Add the length and width and double the result: P = 2(l + w). For 8 by 5 the perimeter is 26.

What is the diagonal of a rectangle?+

The diagonal joins two opposite corners and equals √(length² + width²). For an 8 by 5 rectangle it is √89 ≈ 9.43.

Is a square a rectangle?+

Yes. A square is a special rectangle where the length and width are equal, so all four sides are the same.

What units does the result use?+

The area is in square units of whatever you enter, and the perimeter and diagonal are in the same units as the sides.