Diagonal of a rectangle calculator

The diagonal of a rectangle joins two opposite corners and divides the rectangle into two right-angled triangles. The sides a and b are the two shorter sides of each triangle, and the diagonal is the hypotenuse. Screen sizes for televisions and monitors are also given as the diagonal, just in inches.

How it works

By Pythagoras’ theorem, the diagonal is therefore

$$d=\sqrt{a^2+b^2}$$

A rectangle with sides 4 and 3 has a diagonal of $\sqrt{16+9}=\sqrt{25}=5$.

For the diagonals of other shapes, use the calculators for the diagonal of a square, cube and cuboid.