The diagonal is calculated using the Pythagorean theorem: d = √(w² + h²), where w is the width and h is the height. For example, a rectangle with width 3 and height 4 has diagonal √(9 + 16) = √25 = 5.

For a square with side s, the diagonal is d = s × √2. For example, a square with side 5 has diagonal 5 × √2 ≈ 7.07.

Diagonal measurement is the length of the diagonal, usually given in inches or centimeters. It is commonly used to specify screen sizes (TV, monitor, laptop). A 15.6-inch laptop has a screen diagonal of 15.6 × 2.54 = 39.6 cm.