Rectangle Diagonal Calculator
Calculate the diagonal length of a rectangle from its width and height using the Pythagorean theorem. Enter width and height to instantly find the diagonal distance across the rectangle.
How to use this tool
- Enter width and height in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your diagonal and the full breakdown beneath it.
Formula
d = sqrt(w^2 + h^2)
How it works
The diagonal of a rectangle with width w and height h is d = √(w² + h²).
Worked example
3 x 4 rectangle
- d
- =
- s
- q
- r
- t
- (
- 9
- +
- 1
- 6
- )
- =
- s
- q
- r
- t
- (
- 2
- 5
- )
- =
- 5
- .
Common mistakes to avoid
- Adding width and height before squaring instead of squaring each dimension separately — the correct formula is sqrt(w squared + h squared), not sqrt((w+h) squared).
- Forgetting that the diagonal is longer than either side but shorter than w + h, which is a useful sanity check for your result.
- Swapping diagonal with perimeter: the perimeter is 2(w+h), a completely different calculation.
Key terms
Frequently asked questions
- Does the diagonal of a rectangle bisect the angles?
- Only if the rectangle is a square. In a non-square rectangle the diagonal does not bisect the corner angles equally.
- If I know the diagonal and one side, can I find the other side?
- Yes: missing side = sqrt(diagonal squared minus known side squared). This is just rearranging the Pythagorean theorem.
- Are both diagonals of a rectangle the same length?
- Yes. A rectangle's two diagonals are always equal in length and bisect each other at their midpoints.