Get the height wrong and every movie is a neck workout. Get it right and you forget the TV is mounted at all. The rule is simpler than the internet makes it.
The rule
The center of the screen should sit at about eye level from where you actually sit. For most people on a normal sofa that puts the center around 42 to 48 inches off the floor, but your seat height and the screen size shift it, so measure from your own couch rather than trusting a fixed number.
Bigger screens sit a touch lower
A large TV has more picture above and below center, so you can mount it a little lower than a small one and still see all of it comfortably. Aim the center of the screen at eye level and it works out.
Why fireplaces break the rule
Over a fireplace the mantel forces the TV up, above comfortable height. That is exactly why the pull-down mount exists: it drops the screen to eye level when you watch. If you are set on the fireplace, read mounting a TV over a fireplace.
Bedrooms and standing rooms
From a bed you look slightly up, so a tilt mount a little higher than living-room height works. In a gym, bar or kitchen where people stand or glance up, higher is fine and a tilt angles the screen down toward them.
Not sure where yours should land? We hold the screen up and set the height with you before we drill, on every TV wall mount.