Dividing gross impressions by the appropriate population base and multiplying the answer by one hundred is another way to calculate GRPs. Impressions Impressions are another way to express audience size. Each exposure to your advertising is an impression. Multiplying your target ratings times the applicable population universe, and dividing by one hundred, equals target impressions. Gross impressions are similar to gross rating points in that a target prospect is counted each time they are exposed to your advertising message.
Net impressions are a form of reach .
The target prospect is counted only on the first exposure. A medium's rating is its audience size expressed as a percent of the measured universe. One rating point equals one percent of the base. Sometimes ratings are called percent coverage. If you use a medium just once, its rating and its reach are the same thing.