What is reach in advertising?

Reach counts people. Impressions count showings. If one person sees your ad five times, that is one person reached and five impressions.

An impression is counted each time an ad is shown. It counts showings, not people. So for the same campaign and period, impressions should come out at least as high as reach: everyone reached was shown the ad at least once. If a report shows otherwise, ask how each number was produced.

How often the same person saw the ad, on average, is frequency: impressions divided by reach.

Meta says reach is calculated using sampled data¹. It is not a count of every person. It is a figure worked out from a sample.

So read a reach number as "about this many". When you compare channels, ask how each one counted it before you put the numbers side by side.

It does not tell you whether anyone bought. A board may ask "how many people saw it?". Reach answers that, roughly. The next question is "did it change anything?", and reach cannot answer it.

For that you need a comparison. Lift is the difference in results between a group exposed to ads and a comparable control group. Brand lift measures the change in brand metrics, such as ad recall, between people who saw a campaign and a control group, usually by survey.

Also ask who was reached. People who already buy from you count in reach too.

For where reach fits into channel choices by margin, read paid media strategy for ecommerce.