Under every post you publish, LinkedIn shows you two numbers that founders routinely treat as one. They are not the same, and the difference between them is the most useful thing on the panel.
The two definitions, in LinkedIn's words
Impressions are, in LinkedIn's own phrasing on its post analytics help page, the number of times your post was displayed on screen.
Members reached is the number of distinct members and Pages that saw your post.
So impressions count screens and members reached counts people. If one person scrolls past your post on Monday, comes back to it from a notification on Tuesday, and opens it again from your profile on Wednesday, that is one member reached and three impressions.
Why the gap is the signal
Divide impressions by members reached and you get the average number of times a single person saw the post.
Close to 1.0 means almost everyone who saw it saw it once. The post went out, it was scrolled past, that was the end of it.
Meaningfully above 1.0 means people came back. They returned from a notification because somebody they know commented, or they opened it a second time from your profile, or they scrolled past and then went looking for it. That is the shape of a post that started a conversation rather than one that merely got distributed.
This is why members reached is the better headline number for a founder. Impressions can rise purely because the post sat in more feeds. Members reached rising means more distinct humans, and the ratio between them tells you whether those humans did anything.
A caveat worth keeping: a high ratio on a post with tiny reach is mostly noise. Two people returning four times each is not a signal about anything. Read the ratio only once the post has reached a few hundred people.
Neither number means much on its own
There is no good impressions number, and anyone who gives you one without asking how many followers you have is guessing. Reach scales with the audience LinkedIn can show a post to, which is mostly your follower count plus your network's, so the same post is a triumph for one account and a disappointment for another.
Two honest comparisons:
Against your followers. Divide reach by your follower count. A post that reaches a fraction of your own audience is telling you something about the post, not about LinkedIn.
Against your own history. The only benchmark you can actually see. Take your last ten or twenty posts, work out your own median, and read each new post against that. A post at twice your median did something; a post at half did something too, and it is worth knowing which.
And judge after two days, not two hours. Most of a post's reach arrives in the first 48 hours, so an early read is mostly a read of who happened to be online.
Why nobody can build this for you automatically
LinkedIn's API does not return a member's own post analytics. Not to us, not to any tool. Anything claiming to show you your impressions has either had you paste them in, or is reading the numbers from the page in your own browser, or is showing you public engagement counts and calling them reach.
Sonar takes the second route, with a browser extension that reads your own analytics in your own browser and reports a number only when LinkedIn actually labelled it. If the label is ambiguous or the period is unknown, it drops the metric rather than guessing, which is a rule we adopted after an earlier version reported profile views as impressions.
The point of holding your own numbers is not the dashboard. It is that "this post earned twice your median" is a fact you can write the next post from, and "it got 4,000 impressions" is not.
More on what each number means, with LinkedIn's own sources, in what are LinkedIn impressions.