We can measure the viewing distance from where we sit to our TV.
Using this isosceles triangle calculator (meaning the other two angles away from us are the same), we can use the height and vertex angle selection at the bottom of the drop down to calculate the TV screen width for any given viewing distance and viewing angle.
For example, say the viewing distance is 108″ (9 feet x 12 inches/foot), and we want to solve for a 30 degree viewing angle, that gives a TV width (base a) value of 57.87 > 58″.
Add an inch on either side for the TV bezel (frame) and that gives us 60″ wide. 60″ x (9/16) = 33.75 > 34″ height.
- 20 degrees = 2.5x (i.e. 108″/2.5 = 43.2″ > 43″ screen class size)
- 30 degrees = 1.6x (i.e. 108″/1.6 = 67.5″ > 68″ screen class size)
- 40 degrees = 1.2x (i.e. 108″/1.2 = 67.5″ > 90″ screen class size)