Find the diagonal of a rectangle, square or box from its side lengths, with aspect ratio.
diagonal
How to use this tool
Type the two sides of the rectangle in the same unit.
If you also enter a depth, the calculator gives the 3D space diagonal through the box.
This is the straight line between opposite corners — the same measurement used for TV and monitor sizes.
Length divided by width, written as _ : 1, for example 16:9 screens are 1.78 : 1.
Formula & worked example
A 3 × 4 rectangle has diagonal √(9 + 16) = √25 = 5. A 5 × 12 rectangle has diagonal 13. A 3 × 4 × 12 box has space diagonal √(9 + 16 + 144) = 13.
Limitations — read this before you rely on it
Both sides must use the same unit — mixing centimetres and inches gives a meaningless diagonal. For a screen, the advertised size is the diagonal of the display panel itself, not including the bezel.
Frequently asked questions
The line joining opposite corners, found with Pythagoras: square both sides, add them, and take the square root.
Diagonally across the screen panel, in inches. A 55-inch TV measures 55 inches from one corner to the opposite corner.
Length divided by width. A 16:9 screen has a ratio of 1.78 : 1.
Add the squares of length, width and depth, then take the square root — the 3D version of Pythagoras.
Yes: unknown side = √(diagonal² − known side²).