TV Size Converter

TV size is measured diagonally across the screen.

For standard 16:9 TVs:

• Width = Diagonal x 0.8716
• Height = Diagonal x 0.4903

For ultrawide 21:9 screens:

• Width = Diagonal x 0.9191
• Height = Diagonal x 0.3939

For classic 4:3 screens:

• Width = Diagonal x 0.8
• Height = Diagonal x 0.6

The 4:3 pair are the only ones that come out as round numbers, and they are the 3-4-5 triangle every schoolchild meets. Every factor here is just the side divided by the diagonal of the ratio: for 16:9 the diagonal is the square root of 16² plus 9², which is 18.358, so the width factor is 16 divided by 18.358.

Common TV sizes (16:9):

The centimetre columns are converted from the exact diagonal, not from the rounded inch figure beside them. Convert 47.9 rather than 47.941 and you land on 121.7 instead of 121.8, which is where most published versions of this table pick up a tenth of a centimetre that is not really there.

Math behind it: Using Pythagorean theorem with a 16:9 ratio.

The thing to watch is that TVs are sold by their diagonal alone, which hides how big the screen actually is. A 50-inch TV is about 43.6 inches wide, not 50, and that’s before the stand or bezel. It also means the jump between sizes is bigger than the numbers suggest: moving from a 55-inch to a 65-inch isn’t a 10-inch upgrade in area, it’s roughly 40 percent more screen, because area grows with the square of the diagonal.

This matters for two practical things. First, whether the set physically fits your stand or wall, so measure the width and height, not the diagonal. Second, how far back to sit: for a 4K set the useful band runs from about 0.75 to 1.5 times the diagonal, and that lower figure is not a comfort limit, it is the point at which you would start to make out individual pixels. On a 55 inch screen that is roughly 3.4 to 6.7 feet. Most people sit well past the far end of that and never see the resolution they paid for.

One thing the diagonal hides completely: aspect ratio

A 34 inch ultrawide monitor and a 34 inch television are not the same amount of screen. At 21:9 the ultrawide is 31.2 inches wide and 13.4 high, giving 419 square inches. At 16:9 the television is 29.6 by 16.7, giving 494 square inches, almost a fifth more glass from the same advertised number. The ultrawide feels bigger because it is wider, and it is smaller because it is much shorter. Anyone cross-shopping a monitor against a TV on diagonal alone is comparing the wrong number.