WorldCalcs

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

Area

40

A = l × w

Perimeter

26

P = 2(l + w)

Diagonal

9.433981132

d = √(l² + w²)

What is a rectangle calculator?

A rectangle calculator finds the key measurements of a rectangle from its length and width: the area (space inside), the perimeter (distance around) and the diagonal (corner to corner). Rectangles are everywhere, from rooms and screens to sheets of paper and plots of land, so these are some of the most-used measurements in everyday maths, building and design.

How it's calculated

For length l and width w: the area is l × w, the perimeter is 2 × (l + w), and the diagonal uses the Pythagorean theorem, d = √(l² + w²), since the diagonal splits the rectangle into two right triangles. A square is just a rectangle where length and width are equal.

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, about 9.433981.

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 I find the area of a rectangle?+

Multiply length by width: A = l × w.

How do I find the perimeter of a rectangle?+

Add the length and width and multiply by 2: P = 2(l + w).

How do I find the diagonal of a rectangle?+

Use d = √(length² + width²), from the Pythagorean theorem. See our Pythagorean Theorem Calculator.

What is the difference between area and perimeter?+

Area is the space inside (square units); perimeter is the distance around (linear units).

Is a square a rectangle?+

Yes, a square is a rectangle with all sides equal.

What units does the area use?+

Square units of whatever you entered; perimeter and diagonal stay in the original units.

How do I find a missing side from the area?+

Divide the area by the side you already know.

Why does the diagonal use the Pythagorean theorem?+

Because the diagonal, length and width form a right triangle.

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