How to Measure a Computer Monitor's Screen Size

A computer monitor’s advertised screen size is the distance from one corner of the viewable display area to the diagonally opposite corner. It is not the width of the monitor, and it does not include the plastic bezel or the outer casing. Measurements are usually quoted in inches, even in the UK, although you can measure in centimetres and convert.

For a standard rectangular monitor, put the zero of a tape measure at the bottom-left inside edge of the screen and read the distance at the top-right inside edge. That single number is the screen size. If the reading is 68.6 cm, the monitor is sold as 27 inches.

You do not need special tools, but a few basics make the job quicker and more accurate:

• A rigid tape measure or a long straight ruler. A metal tape is easier to hold straight across a large screen.
• A monitor that is switched off or showing a plain background, so the edge of the active area is easy to see.
• Something to write the measurement down; phone notes are fine.
• A calculator if you are converting centimetres to inches or using width and height.

You do not need to remove the monitor from its stand or take it off the desk for a diagonal measurement. A second person helps on a large or wall-mounted screen, mainly to keep the tape level and stop it slipping.

Sometimes the diagonal is awkward to reach, especially on a wall-mounted monitor or one inside a cabinet. You can calculate it from the viewable width and height using Pythagoras’ theorem: diagonal = √(width² + height²). Measure both in the same unit.

For example, a viewable area of 60 cm wide by 34 cm high gives 60² + 34² = 3,600 + 1,156 = 4,756. The square root of 4,756 is about 69 cm, or 27.2 inches. That is close enough to identify a 27-inch monitor; advertised sizes are rounded.

These screens need the same diagonal measurement, but the handling is slightly different:

• Curved monitors: measure the straight-line distance between the two opposite corners of the active area. Do not run the tape along the curve; that path is longer than the screen’s quoted diagonal.
• Ultrawide monitors: the diagonal is still the quoted size, but it tells you little about desk fit. A 34-inch ultrawide is much wider and shorter than a 34-inch 16:9 screen.
• Borderless or very thin bezel designs: the active area can run almost to the edge, but there is still a narrow border. Start at the lit edge, not the outer seam of the casing.
• Touchscreens and glossy panels: hold the tape by the sides and avoid dragging the metal hook across the coating.

Screen size is useful for comparing monitors, but it is not a complete fit guide. If you are checking whether a monitor will fit under a shelf, on a desk or on a wall arm, measure the overall width, height and depth, including the stand and the rear ports. The diagonal can stay the same while the stand and casing change a lot.

• Desk fit: measure the space available and the monitor’s widest point, usually the stand base or the screen’s outer edge.
• Wall or arm mounting: check the mounting pattern and the monitor’s weight separately; diagonal size does not tell you either.
• Replacing a screen: compare the viewable width and height, not just the diagonal, if you want the same physical image area.

Once you have a number, compare it with the model’s specification. A monitor sold as 27 inches should measure close to 68.6 cm diagonally. Small differences of a few millimetres are normal because quoted sizes are rounded and the visible area can vary slightly between designs. A difference of 2–3 cm or more usually means the tape included the bezel or the casing.