Three of the most common rectangle questions — area, perimeter, and diagonal length — answered together from just length and width.
How it works
Multiplying length by width gives area; doubling their sum gives perimeter; the Pythagorean theorem applied to length and width gives the diagonal.
What this does not include
This does not include a non-rectangular parallelogram (where the angles aren’t 90 degrees) — this calculator specifically assumes right angles throughout.
How to use this calculator
- Enter the length and width.
A worked example
A rectangle 5 × 8: area = 40, perimeter = 2(5+8) = 26, diagonal = √(5²+8²) = 9.433981.
What the variables mean
| Variable | Meaning |
|---|---|
| Length, width | The rectangle’s two dimensions |
Edge cases worth knowing
A square is just a rectangle where length equals width — this calculator’s formulas apply unchanged, with the diagonal formula simplifying to side × √2 in that special case.
A length of zero collapses the rectangle to a line with no area, so the calculator declines to show a result.
Frequently asked questions
What’s a practical use for the diagonal measurement?
Checking that a rectangular frame, doorway, or foundation is actually square — if the two diagonals of a rectangle aren’t equal, the corners aren’t true right angles.
Does a square use the same formulas?
Yes — a square is just a rectangle where length and width happen to be equal; all three formulas work unchanged.
Why does perimeter use “2 times the sum” instead of adding all four sides?
Because a rectangle has two pairs of equal sides — doubling the sum of one length and one width is a shortcut for adding all four sides individually.
