Where the width comes from
A screen is a rectangle with a stated diagonal and a stated shape. Call the ratio w to h; the diagonal of that shape is the square root of w squared plus h squared, so the real width is the diagonal times w divided by that root, and the height is the diagonal times h divided by the same root. For 16:9 the root is the square root of 337, about 18.358, which makes the width 0.8716 times the diagonal and the height 0.4903 times it.
So a 27 inch 16:9 panel is 23.53 inches wide and 13.24 high. A 55 inch television is 47.94 by 26.96, a 65 inch is 56.65 by 31.87, and a 75 inch is 65.37 by 36.77. Those width figures are the ones to hold against a tape measure, because the diagonal is a distance nothing in your room runs along.
Why the same diagonal gives different screens
Fix the diagonal and the area is not fixed with it. Area is maximised by the square and falls away as the rectangle gets longer, because a stretched rectangle spends more of its diagonal going sideways and less of it enclosing anything. At a 27 inch diagonal a 1:1 screen has 364 square inches, a 4:3 has 350, a 16:9 has 311, a 21:9 has 274 and a 32:9 has just 197 — barely more than half the square.
The practical version: a 34 inch ultrawide and a 27 inch 16:9 monitor have almost exactly the same height. The ultrawide is about eight inches wider and that is the entire difference. It is more monitor for side-by-side windows and no more monitor at all for a long document, a tall image or a code file. People replace a 27 with a 34, expect it to feel bigger in every direction, and are surprised when the vertical space is unchanged.
The ratio on the box is usually rounded
Very few screens sold as 21:9 are actually 21:9, which would be 2.333:1. The common 3440 by 1440 panel is 2.389:1, and 2560 by 1080 is 2.370:1. Both get marketed as 21:9 because it is a tidier name than either. The error in the width this produces is around two percent, roughly half an inch on a 34 inch screen — enough to matter for a tight alcove and not enough to matter for anything else. If you need the exact figure and you know the native resolution, use the resolution as the ratio, since square pixels make the pixel ratio the true panel ratio.
Measuring the space rather than the screen
Every figure on this page describes the glass. The product is bigger, and by an amount no formula knows: bezels run from about a quarter inch on a modern monitor to an inch on a budget television, stands are frequently wider than the panel and always deeper, and a set pushed against a wall still needs a few inches behind it for the power cord to leave without being crushed. The bezel allowance here covers the first of those and none of the others.
Before buying to fit a specific opening, pull the manufacturer's dimension drawing, which gives the assembled width, height with and without the stand, and depth. Then measure the opening at three heights rather than one, since alcoves and cabinets are rarely parallel. If the numbers leave under an inch of clearance, treat that as not fitting.
Questions people ask
How wide is a 65 inch TV?
At 16:9, a 65 inch television is 56.65 inches wide and 31.87 inches high — call it 4 feet 8-5/8 by 2 feet 7-7/8. Add the bezel and it is around 57 to 58 inches across the product, and the stand feet may be set wider still. That is the number to check against a media unit or an alcove. The same 65 inch diagonal in 21:9 would be 59.9 inches wide and only 25.7 high, though televisions in that shape are rare outside projector screens.
Is a 34 inch ultrawide bigger than a 27 inch 16:9?
In width yes, in height barely, and in total area by about 33 percent. A 34 inch 21:9 is 31.4 by 13.5 inches; a 27 inch 16:9 is 23.5 by 13.2. The heights are within a third of an inch of each other, so the ultrawide adds eight inches of horizontal room and nothing vertical. Whether that is an upgrade depends entirely on what you look at. Two windows side by side, clearly yes. A single long page, no — for that you want a taller screen, which means a larger 16:9 or a 16:10.
Why does my screen measure smaller than its stated size?
Measure corner to corner on the glass only, not across the bezel, and diagonally rather than along an edge. Most complaints resolve there. Beyond that, a small discrepancy is normal: manufacturers sometimes state a rounded class size rather than the exact panel, so a screen sold as 27 inch may measure 26.9 or 27.1. A discrepancy of more than about half an inch usually means the bezel got included in the measurement, or the panel is a different class than advertised.
Does this tell me the resolution or how sharp the screen is?
No, and deliberately. Physical size and pixel count are independent — a 27 inch panel exists at 1080p, 1440p, 4K and 5K, all the same physical object with wildly different sharpness. What this page gives you is the shape and the footprint. For pixels per inch, dot pitch and whether individual pixels are resolvable from your chair, the PPI calculator on this site takes a resolution and a diagonal and works that out.