A lot of consumers think that a television's screen size is the width of the screen from left to right. But in reality, it is a diagonal measurement.
Position the measuring tape
Get a measuring tape. Place one end at the lower-left corner of the television screen. Have someone hold the other end at the upper-right corner of the screen.
Mark the distance
Mark the distance. If the tape is at 42 inches, for instance, the TV has a 42-inch screen. This size is what retailers mean when they say, "42-inch TV."
Identify the screen ratio
A standard TV screen ratio is 4:3, but a wide-screen ratio is 16:9, according to Radio Shack. The standard ratio means that the screen has 4 inches of width for every 3 inches of length.