Rectangle Diagonal Calculator

About this calculator

A rectangle’s diagonal — the straight line connecting two opposite corners — follows directly from the Pythagorean theorem, since the diagonal is the hypotenuse of the right triangle formed by the rectangle’s width and height: d = √(w² + h²). Once width and height are known, area (w × h) and perimeter (2(w + h)) follow just as directly, giving a complete geometric picture of the rectangle in one calculation.

Builders and framers use the diagonal measurement to “square” a rectangular layout — a foundation, deck, or wall frame is truly square only when both diagonals measure equal length, a much more reliable check than trusting the corner angles alone. It’s also the exact math behind reading a TV or monitor’s advertised screen size, since manufacturers always specify screen dimensions as the diagonal measurement rather than width or height individually, and it’s used in furniture and doorway fitting to confirm a rectangular object will physically pass through a diagonal opening.