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How to Measure TV Size for Your Room

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1THX: the biggest number

THX's own recommendation is a 40-degree view angle, which they translate to consumers as dividing the screen's diagonal measurement by 0.84 to get the ideal viewing distance, roughly 1.2 times the diagonal. It is the most demanding of the common standards and the one home-theater enthusiasts quote most.

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2SMPTE 30 and the old 2.5x rule

SMPTE's widely quoted figure is a 30-degree field of view, which works out to about 1.63 times the screen's diagonal for a 16:9 set. The older, simpler rule many manufacturers and retailers still cite is to multiply the diagonal by 2.5, a noticeably more conservative (smaller-screen-for-the-distance) number.

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3Closer is fine now

CNET's more recent take is that high-resolution 4K content holds up at a closer 1.5 times the diagonal, since there's enough pixel detail that sitting closer doesn't make the picture look worse the way it would on an old 1080p set. A 4K television panel is 3840 by 2160 pixels, four times the pixel count of 1080p, which is exactly what buys the closer seat.

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4The eye chart is the real limit

Underneath all four numbers is the same idea: the human eye can resolve about one arcminute of detail, so at some distance you simply can't see any more resolution than the screen offers. A 1080p set stops paying you back in detail past roughly a 31-degree view angle; 4K pushes that number much further away. That's the textbook definition of 20/20 vision: resolving two points one arcminute apart, about 60 pixels per degree of view.

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5Closer has a cost the math doesn't show

A 2023 study of 114 adults found the people with eyestrain symptoms weren't using smaller fonts, they were sitting closer: 56 centimeters from a computer screen versus 62 for the comfortable group, with a 60-centimeter-or-more preference showing up in earlier work too. Screens scale, so the lesson for a TV is the same one the standards give: pick the distance first, then the size, not the other way round.

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