What is the ideal sitting distance from a television? It depends.
There are many articles on the Internet trying to answer this question, but nearly all of them don’t consider the actual size of the image being watched. They only consider the physical size of the television screen.
A modern TV has a physical screen aspect ratio of 16:9 (i.e. 1.78:1). But many movies are shot in a wider aspect ratio (such as widescreen 1.85:1 or CinemaScope 2.35:1) and displayed with letterboxing (i.e. black bars at the top and bottom of the screen). Old television shows and very old movies were shot with a 4:3 (i.e. 1.33:1) aspect ratio and are also displayed with letterboxing (i.e. black bars at the sides of the screen).
Some answers are based on resolution: recommending different distances for a standard definition TV, high definition TV or a “4K” UHD TV. But that is wrong, since you want to watch the whole image and not individual pixels. Consider two photos that are the same physical size, one is from a low-resolution newspaper and the another is from a high-resolution printer: you don’t hold the high-resolution photo closer to look at it.
A better answer is from John Bishop, when he was interviewed on the Home Theatre Geeks podcast.
He recommends sitting between 1.5 to 3 times the height of the image from the screen.
He is a personal cinema architect, and his answer is based on commercial cinema standards. He says the height of the image is what matters—not the physical dimension of the screen. Therefore, the ideal distance changes depending on the aspect ratio of the content being viewed. If you don’t want to move the seat when the content changes, you can probably find a good fixed position somewhere in that 1.5 to 3 times range.
This is much closer to the screen than the other recommendations. But this is because it is replicating the immersive movie experience inside a cinema. For other types of content, or personal preference, you might to sit further back. But consider this rule when figuring out the viewing distance you want to use.