Field note 04

When a TV, monitor, laptop or phone is described as “55 inches” or “15.6 inches”, that number is the diagonal of the visible picture, from one corner to the opposite corner. It does not include the frame (bezel) or the stand.

Measure it in three steps

1. Turn the screen on with a bright image, so you can see exactly where the picture starts.
2. Run a tape measure from the bottom-left corner of the picture to the top-right corner. Keep the tape straight and do not press on the panel.
3. If your tape is metric, divide by 2.54 to get inches, or use the cm to inches converter.

A 55-inch TV has a 139.7 cm diagonal; a 24-inch monitor, 61 cm.

Width and height from the diagonal

The diagonal alone does not tell you whether a TV fits a cabinet. For the common 16:9 shape, width ≈ diagonal × 0.872 and height ≈ diagonal × 0.490. A 55-inch 16:9 TV is therefore about 121.8 cm wide and 68.5 cm tall, before adding the frame. The screen size and PPI calculator does this for 16:9, 16:10, 4:3 and 21:9.

Laptops and phones

Laptop screens are usually 13.3, 14, 15.6 or 16 inches; measure the same way with the lid open. Phones have rounded corners and notches, so the advertised diagonal is that of the full rectangle; your measurement may come out slightly smaller.

Why you can't measure it with the on-screen ruler

Browsers know a screen's resolution but not its physical size, which is why the on-screen ruler asks for a quick calibration with a bank card. Once calibrated it is accurate for objects you place on the screen, but it cannot see beyond its own edges.