How to measure a TV screen size: diagonal, width and height

What the number on the box really means

The size printed on a TV box is the length of the screen diagonal: from one bottom corner to the opposite top corner, measured across the lit panel only. It does not include the plastic frame, and it says nothing about how wide or tall the set is. Two TVs with the same diagonal can differ by a few centimeters in overall width because one has a thicker bezel.

Because the number is a diagonal, it is longer than either side of the screen. On a standard 16:9 widescreen the width is about 87% of the diagonal and the height about 49%. That is why a 55-inch TV (139.7 cm diagonal) is 121.8 cm wide and 68.5 cm tall, not 139.7 cm wide.

How to measure the screen yourself

• Switch the TV on or look at the glass, so you can see where the picture area ends. Ignore the frame.
• Hook a tape measure on the bottom-left corner of the lit screen and pull it to the top-right corner. That number is the diagonal.
• Divide by 2.54 if your tape is in centimeters and you want inches (or multiply by 2.54 if you measured in inches). The inches to cm converter does this instantly.
• Measure the width on its own, from the left edge to the right edge of the picture, and the height from the bottom edge to the top edge. For a 16:9 TV the width should be 1.78 times the height.
• To fit a TV into a space, measure the outside of the frame as well, because that is the number a cabinet or a wall bracket cares about.

The maths: from diagonal to width and height

A 16:9 screen is a right triangle with sides in the ratio 16 : 9. The diagonal is therefore the square root of 16² + 9², which is about 18.36 units. Divide 16 by 18.36 and you get 0.8716, the share of the diagonal that is width; divide 9 by 18.36 and you get 0.4902, the share that is height.

So width = diagonal × 0.8716 and height = diagonal × 0.4902. For a 43-inch TV the diagonal is 109.2 cm, which gives a width of 95.2 cm and a height of 53.5 cm. Ultrawide monitors use other ratios (21:9 or 32:9), so the shares change there.

Diagonal, width and height for common TV sizes

Why the real size can differ by a few centimeters

Manufacturers round the diagonal to the nearest marketing size, so a "65-inch" panel may measure 64.5 inches. Curved screens are measured along the chord, not along the curve. A few TVs are sold at non-16:9 shapes, and projectors and monitors follow their own ratios.

If the exact fit matters, for example when a TV has to go into a tight alcove, use the width and height published in the spec sheet for the exact model and treat the figures here as a close planning guide. Our TV size chart lists the same numbers from 24 to 100 inches.

Next steps

Once you know the width, the next question is usually what the TV stands on. Read how wide a TV stand needs to be for the clearance rules, or check popular sizes directly: 32″, 43″, 50″, 55″, 65″.