Reach and impressions get used almost interchangeably in casual conversation about social media, and treating them as the same thing leads to real misreadings of what a post actually did. They measure different things, and the gap between them is itself informative.
The actual definitions
Reach counts unique accounts that saw your post — each account counted once, no matter how many times they saw it. Impressions counts every view, full stop — the same account seeing a post three times counts as three impressions but only one unit of reach.
Impressions is mathematically always equal to or greater than reach, for exactly this reason.
Why the gap between them is itself a signal
A post with reach and impressions close together is being seen mostly once per account — straightforward, single-exposure distribution. A post where impressions significantly exceeds reach means the same people are seeing it repeatedly, often because it's surfacing in multiple places (feed and Explore both, say) or people are revisiting it. Neither pattern is inherently better, but they mean different things: the first suggests broad, single-touch distribution; the second suggests deeper, repeated engagement with a smaller set of people.
Which one actually matters for your goals
For most accounts' real goals — growing awareness, finding new followers, expanding beyond the people who already know you — reach is the more directly useful number, since it answers "how many distinct people did this find" rather than "how many total views did this rack up," which can be inflated by the same small audience seeing it repeatedly.
Where this connects to your other metrics
Reach is the number worth tracking as a trend across posts, not judged in isolation per post — a single post's reach tells you about that post; a reach trend across weeks tells you whether your account's overall discovery is improving. Combine this with tracking demographics to understand not just how many new accounts you're reaching, but whether they match who you're actually trying to reach.
A quick sanity check worth running
If a post's impressions are dramatically higher than its reach, look at where it's actually appearing — it may be getting repeat views from Explore alongside feed placement, which is a genuinely good sign (multiple algorithmic surfaces finding it worth showing more than once), distinct from a post that simply has high impressions because a small, existing audience is checking back on it repeatedly without any new reach at all.