Rectangle Calculator: Area, Perimeter and Diagonal

Enter the two side lengths in the same unit to see the area, perimeter and diagonal.

Updated October 27, 2026

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Formula

area = length × width
perimeter = 2 × (length + width)
diagonal = √(length² + width²)

Worked example

A rectangle 4 long and 3 wide:

Area = 4 × 3 = 12
Perimeter = 2 × (4 + 3) = 14
Diagonal = √(16 + 9) = √25 = 5

Frequently asked questions

How do I find the area of a rectangle?

Multiply length by width. A 4 by 3 rectangle has an area of 12 square units.

How do I find the perimeter?

Add length and width, then double it. For 4 and 3 that is 2 × (4 + 3) = 14.

How is the diagonal calculated?

The diagonal is the hypotenuse of a right triangle whose legs are the length and width: √(length² + width²). For 4 by 3 it is √25 = 5.

Does a square work too?

Yes. Enter the same value for length and width. A square of side 2.5 has area 6.25, perimeter 10 and diagonal 3.5355.

How we calculate

Rectangle Calculator: Area, Perimeter and Diagonal

Formula

Worked example

Frequently asked questions

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