The target rating point, or TRP, in advertising is calculated by using the gross rating point, or GRP. TRP equals the total GRP multiplied by the ratio of the target audience to the total audience.
If you are trying to calculate the TRP, get your numbers from sources like Alexa, Cision or rate cards in addition to utilizing your own marketing research. Then, determine the ratio of your target audience in regards to the total audience, which would be the GRP. If the GRP is 50 and the ratio of the target audience is 0.10, then the TRP is 5.