Screens are sold by their diagonal, which hides the width, height and sharpness. Enter a size and resolution to get pixel density, the real dimensions, and the distance at which you stop seeing individual pixels.
An eye with normal vision separates details about one arcminute apart. Beyond a certain distance, pixels blur together and extra resolution is wasted. The line shows where that happens for the screen above, and where each kind of screen is normally used.
A wider shape spreads the diagonal sideways, so a 27-inch ultrawide has less area than a 27-inch 16:9, and a 4:3 has the most. These are drawn to scale at your diagonal.
Browsers report pixels, never millimetres: CSS fixes 1 inch at 96 CSS pixels whatever the real size. Every payment card is the same ISO/IEC 7810 ID-1 size, 85.60 × 53.98 mm. Hold one against the screen and resize the outline until it matches. From that one measurement this page works out your real pixel density, your screen’s diagonal, and how long a CSS “inch” really is here.
How sharp a screen looks depends on how many pixels fit in one degree of your view, which mixes density with distance. An eye that resolves details one arcminute apart (20/20) needs 60 pixels per degree before pixels disappear; sharper eyes need more. Set your eyesight and distance; the figures use the screen from the calculator above.
Every monitor holds an EDID block that the computer reads over the cable: who made it, its physical size, colour primaries and native timing. This builds a valid EDID 1.4 base block for the screen in the calculator, with the native mode timed by VESA CVT reduced blanking. Click a region to decode it, or download the .bin and open it in edid-decode.
The quoted diagonal means different things for different products, and no two layers of the stack store size the same way.
Neighbouring tools, with a typical result for each.