Screen Size Calculator
Enter a diagonal and an aspect ratio to get the real physical width and height of a screen — and see why the same diagonal looks so different at 16:9 versus 4:3.
Width and height at once
Enter the diagonal and the aspect ratio and the calculator returns the physical width and height in the same unit as the diagonal.
Use the native ratio
The split depends entirely on the aspect ratio — 16:9, 4:3, 16:10, or 21:9 — so enter the screen's true ratio for an accurate width and height.
What is a screen size calculator?
From diagonal to real dimensions
A screen size calculator turns the single number printed on the box — the diagonal — into the two measurements you actually care about: the physical width and height of the panel. Screens are sold by their corner-to-corner diagonal, but that figure hides how the size is shared between width and height, which depends on the aspect ratio. Enter the diagonal and the ratio (16:9, 4:3, 16:10, 21:9) and you get the real dimensions in the same unit you typed the diagonal in, so you can check whether a TV fits a wall, a monitor fits a desk, or two differently shaped screens really are the same size.
Enter a diagonal and an aspect ratio to get the exact screen width and height in the same unit instantly.
The diagonal, width, and height form a right triangle, so the aspect ratio and a square root split the diagonal into its two sides.
width = D × w ÷ √(w² + h²)The height uses the same formula with the height component on top: height = D × h ÷ √(w² + h²). The term √(w² + h²) is the diagonal of the aspect-ratio rectangle itself, so dividing by it scales the ratio down to fit your real diagonal. Use any unit for the diagonal and the width and height come back in that same unit.
Suppose you have a 27-inch monitor with a 16:9 aspect ratio.
Find the ratio's diagonal
√(16² + 9²) = √337 ≈ 18.3576 — the diagonal of the 16-by-9 rectangle.
Scale the width
27 × 16 ÷ 18.3576 ≈ 23.5325 in — the physical screen width.
Scale the height
27 × 9 ÷ 18.3576 ≈ 13.2371 in — the physical screen height.
The width and height tell you what the diagonal alone cannot: how the screen really sits in space. For the 27-inch 16:9 monitor above, the 23.53 in width is what has to fit between your desk dividers, and the 13.24 in height is what sits below your eye line. The big insight is that two screens with the same diagonal can be very different shapes. A 27-inch 4:3 panel works out to 21.6 in wide and 16.2 in tall — narrower but much taller than the 16:9 — and its area is larger even though the diagonal is identical. That is why diagonal-for-diagonal comparisons mislead: a widescreen "loses" height to gain width. Multiply width by height to compare usable area, which is the honest way to judge how much picture you are really getting.
The geometry is exact, but a couple of practical points are worth keeping in mind.
Active area only, and keep your unit consistent
This calculator returns the dimensions of the active display area that matches the quoted diagonal — not the outer body of the device, which is a little larger because of the bezel and frame. The width and height come back in whatever unit you entered the diagonal in, so use inches throughout for an inch result, or centimetres throughout for a centimetre result; to convert inches to centimetres, multiply by 2.54.