How to measure a monitor

Measure the glass, corner to corner, diagonally — not the bezel, not the stand, not the width alone. That single instruction is most of what there is to know, and the rest of this page is the part every generic answer skips: how to check your own measurement against the geometry.

Guardrail fact: a genuine 27 inch 16:9 panel measures 59.8 cm across and 33.6 cm tall. If your tape says 62 cm wide, you measured the bezel, not the screen.

Turn a measurement into every other dimension once you have it.

The measurement, step by step

Power off the monitor, or fill it with a plain bright image, so the edge of the actual picture is unambiguous — a black bezel next to a black background is the single most common source of a wrong reading. Measure from the bottom-left corner of the image to the top-right corner, diagonally, in a straight line. Record the figure in centimetres; converting to inches (divide by 2.54) only matters once you want to compare against a class name.

Now check your answer

For a 16:9 panel, divide the measured width in centimetres by 2.2138 and the result should be your diagonal in inches. The same factor run in the other direction gives the reverse table below — the width a genuine panel of each common size actually measures.

A 27 inch, exactly and a 24 inch both have their own full breakdown if either is the size you're actually checking. Ultrawides measure differently: 21:9 uses a different factor, 2.3346, so a 34 inch measures 79.4 cm across and a 38 inch 88.7 cm — a plain 16:9 factor applied to an ultrawide's width gives the wrong size class entirely. Super-ultrawide (32:9) uses 2.4451: a 49 inch measures 119.8 cm across. If your own measurement lands between two rows here, you likely measured slightly off the true corner-to-corner line — diagonal measurements are unforgiving of a few degrees of drift.

The four mistakes

Measuring the bezel. The advertised size is the image only; the bezel adds nothing to it and subtracting it after the fact is guesswork the actual measurement avoids entirely.

Measuring horizontally and calling it the size. Width alone is not the diagonal, and the two numbers are only related once you also know the aspect ratio — reporting a width as if it were the size skips a step that changes the answer.

Measuring a curved screen around the curve. A curved panel's glass follows an arc rather than a flat plane, and a tape measure laid along that arc reads long — on a typical 1000R curve, by roughly a centimetre. Measure the straight chord between the two corners instead, not the path the glass itself takes.

Including the stand. The stand and its base are not part of the diagonal under any convention; a measurement that starts below the panel itself is measuring the wrong object.

Doing it without a tape measure

Windows: Settings → System → Display → Advanced display gives your resolution, not your physical size — the two are unrelated once scaling is involved; the screen resolution checker reads the resolution directly if that's what you actually need. The reliable route to a physical size is the model number, found at Settings → System → About or printed on a label on the back of the panel, checked against that model's own spec sheet.

macOS: → About This Mac → More Info → Displays lists the connected display's model name, which again needs checking against its own spec sheet for the physical size.

Linux: xrandr prints a physical size in millimetres, read from the display's own EDID data. The one caveat worth knowing: EDID reports physical size in whole centimetres only, so anything reading it — including xrandr — is accurate to within about a centimetre, not more precise than that.

Why "27 inch" is a class, not a measurement

The diagonal describes the active area only, and "27 inch" is a class of panels built to roughly that diagonal, not a guarantee of an exact figure. Some manufacturers' own spec sheets footnote a measured diagonal slightly under the class number — check the specific model's spec page rather than assuming every panel sold as a given size measures identically.

What the number is actually for

Desk fit depends on width, not diagonal — a 27 inch and a 27 inch ultrawide (there isn't one; ultrawides are a different width class entirely) would need very different desks if they existed at the same diagonal, which is exactly why width is the number to check against your own space, covered in full on will it fit your desk?. The VESA mounting pattern and monitor-arm weight rating both come from the model's own spec sheet, not from the diagonal. And resolution is worth choosing with the diagonal in mind: at 27 inches, 1440p is 108.8 pixels per inch and 4K is 163.2 — the same pixel grid reads differently depending on the glass it's spread across. See every size at once on the full screen size chart if you're comparing more than one.