WHAT IS GRP AND HOW IT IS CALCULATED?

Gross Rating Point (GRP) is a term used in advertising to measure the size of an audience reached by a specific media vehicle or schedule. It is the product of the percentage of the target audience reached by an advertisement, times the frequency they see it in a given campaign. For example, a TV advertisement that is aired 5 times reaching 50% of the target audience, it would have 250 (GRP = 5 x 50% --) i.e., GRPs = frequency x % reach. To arrive at your total Gross Rating Points, add the individual ratings for each media vehicle you are using.
You can also calculate GRP by dividing your gross Impressions by the population base and multiplying the answer by 100. GRPs are also used by broadcasters to sell their advertising space to potential customers.