Reference library · Media planning
Reach Curve (GRP-to-Reach Conversion)
A reach curve plots cumulative GRPs against the percentage of the target audience reached at least once, and it always bends downward because each new spot increasingly hits people already reached. Planners read it to find the GRP level where added weight stops buying meaningful reach and starts buying frequency instead. It underlies the decision of how many GRPs to put behind a single flight before further spend becomes inefficient.
Formula
Worked example
A national TV plan uses spots averaging r = 5% (0.05) against the target group. With n = 20 spots, GRP = 20 x 5 = 100, and Reach = 1 - 0.95^20 = 64%. Adding another 20 spots (n = 40, GRP = 200) gives Reach = 1 - 0.95^40 = 87%. Doubling GRPs from 100 to 200 only lifts reach from 64% to 87%, a 23-point gain rather than the 64 points a linear reading would suggest, meaning the second 100 GRPs mostly add frequency among people already reached instead of new viewers.
How it is used
Planners rarely compute the binomial estimate above by hand; they pull the curve from panel-based software (Nielsen, Kantar, Telmar) built on actual audience duplication data, since real viewers are skewed toward heavy and light consumption and the true curve flattens faster than the independence assumption implies. The curve sets a practical GRP ceiling per flight or channel, past which budget is better spent opening a second channel or daypart than adding more spots to the first. The recurring mistake is reading the early, near-linear part of the curve and extrapolating it through the rest of the plan, which overstates reach at higher GRP levels.
The common mistake
The most common error is assuming reach scales linearly with GRPs across the whole plan, when each incremental block of GRPs in fact buys less new reach as spot levels rise.