How this calculator works
The area is the product of the two side lengths, and the perimeter is the sum of all four sides. The diagonal joins opposite corners and is the hypotenuse of the right triangle the sides form, so the Pythagorean theorem gives its length. Negative sides are rejected; a zero side is allowed as a degenerate case.
Formula
Area = l × w; Perimeter = 2(l + w); Diagonal = √(l² + w²).
Worked example
A rectangle of 8 by 5 has area 40, perimeter 2 × 13 = 26 and diagonal √(64 + 25) = √89 ≈ 9.4340.
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Frequently asked questions
Which side is the length?
It does not matter: both products are symmetric, so entering 8 by 5 or 5 by 8 gives identical results.
How is the diagonal found?
The diagonal splits the rectangle into two right triangles, so its length is the square root of the sum of the squared sides.
What happens if one side is zero?
A zero side is allowed as a degenerate case: the area is zero, the perimeter is twice the other side, and the diagonal equals the other side.