A Neilsen rating is expressed as a fraction. The numerator is called the “rating”, the denominator is called the “share”.
This is a percentage of all the Americans that own a TV set. In other words, it expresses how many Americans were watching a particular show, regardless of whether or not they had the television turned on.
This measures the percentage of all the individuals who are watching TV during a particular time slot. In other words, while ratings measure percentage of the population whether or not they have the TV sets turned on, shares only express the percentage of all the TV sets that are turned on at a particular time.
So if a show has a 2.2/11 Neilsen rating, this means that 2.2 percent of the entire American population who own TVs watched the show, and of the population that had their TVs turned on, 11 percent of those people were watching your show. For a primetime show, you are mainly looking at the population between the ages of 18-49. There are 135 million Americans in this age range. So when a Neilsen rating is given for a prime time network show, it’s usually a percentage of 135 million people.
Click here to find out how they get their numbers.