Contents
How the rectangle diagonal calculator works
Enter the rectangle’s two side lengths in the same unit. The calculator returns the straight-line distance between opposite corners.
The formula behind the answer
A diagonal splits a rectangle into two right triangles. Its length is the hypotenuse, so the Pythagorean theorem applies:
Here, a and b are the side lengths and d is the diagonal. Keep both inputs in one unit; the answer uses that unit too.
Three quick checks
These familiar triples make it easy to check the calculation.
- 3 by 4 gives 5. This is the classic 3-4-5 right triangle.
- 5 by 12 gives 13. The same relationship works in centimetres, feet, or any other shared unit.
- 1 by 1 gives about 1.414214. A square’s diagonal equals its side multiplied by √2.
For a 120 cm by 70 cm screen, the corner-to-corner distance is about 138.92444 cm. A marketed screen size may differ because manufacturers measure the visible panel rather than the outer frame.
Measure the rectangle, not an approximation
The formula assumes four right angles. If a frame is skewed, its two diagonals can differ and the result describes the ideal rectangle made from the entered sides.
- Measure both sides along straight edges.
- Do not mix centimetres and metres in the two inputs.
- Keep extra decimals when the diagonal will be used for cutting or fitting.
Questions about rectangle diagonals
The short answers below cover the cases that most often cause a wrong measurement.
Can I find the diagonal from area alone?
No. Rectangles with the same area can have different proportions and different diagonals. You need both side lengths or equivalent information.
Is the diagonal always longer than either side?
Yes, for a rectangle with two positive side lengths, the diagonal is longer than each individual side.
Does the formula work for a square?
Yes. Set both sides to the same value; the diagonal equals that value multiplied by the square root of 2.
Can I use inches or feet?
Yes. Choose the unit and enter both sides in that same unit. The result appears in the selected unit.
Why might a measured diagonal be different?
Real objects may be out of square, measurements may include a frame, or the side lengths may have been rounded.
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