How to measure a screen size

Screen size is one measurement: the diagonal of the viewable picture, from one lit corner to the opposite one. Everything else — class sizes, bezels, “true” size claims — follows from that definition.

The short answer

Measure from the top-left corner of the picture (not the frame) to the bottom-right corner, in inches. That number, rounded, is the screen's size. Do not include the bezel, and do not measure width or height and call it the size — a screen that is 27 inches wide would be a 31-inch-class screen diagonally.

Step by step

1. Turn the screen on and show something bright — the lit area's edge is much easier to find than the panel edge on a dark screen.
2. Place the tape at one corner of the picture, just inside the black border that surrounds the image on most displays.
3. Run it diagonally to the opposite corner and read the length in inches (or centimetres — divide by 2.54).
4. Compare with the class: expect a small shortfall. A “24-inch class” monitor commonly measures 23.8″; a “65-inch class” TV is usually within a few millimetres of 65″ of picture.

Why “class” sizes exist

Panel makers cut glass in exact sizes that rarely land on whole inches, and marketing prefers whole numbers — so the industry sells classes: 23.8″ and 24.5″ panels both sit in the “24-inch class” in some catalogues, though most brands now print the true diagonal (23.8″) next to the class name. The same applies at TV sizes: check the fine print for “viewable diagonal” if two TVs both claim 55″ but one looks smaller in the shop. When you measure yours, judge it against the class tolerance — within about half an inch at monitor sizes — rather than expecting the exact headline number.

What the measurement leaves out

• Bezel and cabinet: add them from the product's “dimensions without stand” spec when checking furniture.
• Curved screens: the diagonal is still the straight corner-to-corner line, not the arc length along the curve.
• Projectors: the “size” is whatever diagonal the projected image has at your throw distance — it changes as you move the projector.