What Is an Ad Impression? A Plain-English Guide

Open any advertising report and the first number you'll meet is impressions. It sounds simple, yet it's one of the most misread metrics around, because people assume it means the number of people who looked at an advert. It doesn't.

I've pulled this together from Google Ads Help, which defines these terms for the platform most business owners start with. Here's what an impression is, what it isn't and how it fits with clicks and viewability.

Ad impression meaning in plain English

Google Ads Help defines an impression as being counted each time your ad is shown on a search result page or another site on the Google Network. It says the same thing in short form elsewhere: an impression is counted each time your ad is served.

So an impression is a count of showings. It's not a count of people, and it doesn't say whether anybody noticed the advert. Think of leaflets pushed through letterboxes. The number delivered tells you how many landed on doormats. It doesn't tell you how many were read.

What counts as one ad impression

The count is fairly literal. Each time the advert is shown, that's one impression. Google's help page adds that sometimes only part of an advert appears. Its example is Google Maps, which may show just your business name and location, or your name plus the first line of the advert text. That still counts.

• If the same person sees your advert three times, that's three showings.
• In an account, you'll often see the word shortened to Impr.

The first point follows from the definition, which counts showings and not individuals. That's why impressions are a good measure of how widely an advert is being delivered, but a poor measure of how many different people you've reached.

Impression vs click

A click is a separate event: someone doing something with the advert. The ratio between the two has its own name. Google Ads Help defines click-through rate, or CTR, as a ratio showing how often people who see your ad end up clicking it. It's calculated as clicks divided by impressions, so 5 clicks from 100 impressions is a CTR of 5 in 100.

Term	What it counts	What it tells you
Impression	Each time the advert is shown	How often it was delivered
Click	Each time someone clicks it	How many acted on it
Click-through rate	Clicks divided by impressions	How often a showing led to a click

Google notes that what counts as a good CTR depends on what you're advertising and on the network. There isn't one number to aim for.

Impressions vs viewable impressions

Because an ordinary impression only means the advert was served, the industry developed a stricter measure. Google's Active View help page defines a viewable impression as one where at least half of the advert's area is on screen for one second for display adverts, or two seconds for video.

The same page sets out how the terms nest. Viewable impressions are a subset of measurable impressions, and measurable impressions are a subset of total impressions. Measurable impressions are those on sites or apps where Active View can capture viewability data. The viewable rate is viewable impressions divided by measurable impressions.

• Total impressions: every time the advert was served.
• Measurable impressions: served where viewability could be captured.
• Viewable impressions: measurable ones that met the on-screen test.

Even a viewable impression is a chance to be seen, not proof of attention. Someone might have the page open but be looking elsewhere.

How impressions are counted away from the screen

The idea of counting showings isn't limited to the web. Posters and billboards have an equivalent, but there's no software logging each viewing, so audiences are estimated from surveys and travel data instead. I explain how that works in my guide to what out-of-home advertising is.

How to read an impressions number sensibly

A big impressions number looks impressive in a report, but it only becomes useful when it's read alongside other numbers. Here are some questions worth asking:

1. How many of those impressions were measurable and viewable?
2. What was the click-through rate, and how does it compare with earlier campaigns for the same advert?
3. Were the impressions spread across lots of people, or the same few people seeing the advert again and again?
4. What did the clicks lead to once people reached the website?

Impressions answer the first question in the chain, which is whether the advert was delivered at all. If the number is very low, that's worth investigating. Google's help centre has a separate article on fluctuating impressions for exactly that situation.

Common ad impression mistakes

• Treating impressions as the number of people reached.
• Assuming every impression was seen. A served impression and a viewable one aren't the same.
• Judging an advert on impressions alone, with no look at clicks.
• Comparing impressions between platforms without checking how each one counts.
• Forgetting that partial showings, such as a name and location in a map result, still count.

Writing about advertising metrics

Advertising terms confuse a lot of business owners, so clear explanations are welcome. If measurement is your field, you might think about writing for other publications.

Ad specialists who write can browse advertising sites that welcome outside contributors.

Frequently asked questions

What is an impression in advertising?

It's a count of each time an advert is shown. Google Ads Help says an impression is counted each time your ad is shown on a search results page or other site on the Google Network.

Is an impression the same as reach?

No. Impressions count showings, so one person seeing an advert three times adds three impressions. Reach would count that person once.

What's a viewable impression?

According to Google's Active View help, it's one where at least half the advert's area is on screen for one second for display adverts, or two seconds for video.

How is click-through rate calculated?

Divide clicks by impressions. Google gives the example of 5 clicks from 100 impressions.

Does a partial advert still count as an impression?

Yes. Google's help page says that sometimes only part of an advert appears, such as a business name and location in Google Maps, and it still counts.