About this tool
Calculate diagonals four ways: a rectangle from its length and width, a square from its side, a 3D box from its three edges, or the real width and height of a screen from its diagonal and aspect ratio. Everything is computed in your browser.
The diagonals use the Pythagorean theorem: a rectangle is √(l² + w²), a square is s·√2, and a box's space diagonal is √(l² + w² + h²). For screen sizing with an aspect ratio w:h (for example 16:9), the scale factor is diagonal / √(w² + h²); the width is w × scale, the height is h × scale, and the display area is width × height.
The screen mode is ideal for TVs and monitors: enter a 50 inch diagonal at 16:9 to see it measures about 43.6 by 24.5 inches. The other modes also report area, perimeter, volume or surface area where they apply.