What Is an Impression? How DOOH Counts Them

A DOOH impression is the base measurement unit that expresses how many people a single ad play on a digital out-of-home screen reached. This article explains why an impression and a play are not the same thing, how one play turns into many impressions, the concept of the “impression multiplier,” and how modeled versus measured counting changes the result — with worked examples.

What is the difference between an impression and a play?

Two concepts are often confused in DOOH: the play and the impression. A play is a single showing of an ad on the screen; it is purely a screen-side, technical event. If the screen looped that ad six times in an hour, six plays occurred.

An impression, by contrast, is on the audience side: it is the number of people who had the opportunity to see the ad during that play. This is exactly why one play can correspond to many impressions. A ten-second play in a busy transit corridor reaches the dozens of people who passed through in those ten seconds. The same play at midnight on a quiet street might reach only a handful.

This distinction matters because an advertiser buys reached audience, not plays. A campaign’s value is measured not by how many times it played, but by how many people it reached and at what quality. We cover where impressions sit within DOOH measurement as a whole in the audience measurement pillar.

How does one play turn into many impressions?

Going from a single play to impressions is, logically, a three-step flow.

First the play happens: the ad appears on the screen. Then the people who are within the screen’s catchment at that moment and have the opportunity to see the ad come into the picture. That headcount is not billed directly; it is converted into an adjusted value, the billable impression, according to a rule the industry agrees on. The coefficient that performs this conversion is the impression multiplier.

The multiplier’s job is to reduce the raw passing count to an estimate of “people who genuinely had the opportunity to see.” People walking past sideways, never turning toward the screen, or staying only briefly are accounted for through this adjustment. The multiplier concept is the technical foundation of programmatic DOOH; we examine how it works over OpenRTB in the OpenRTB and impression multiplier article.

What is the impression multiplier, and why does it vary by venue?

The impression multiplier is the number that says how many billable impressions a play corresponds to. The critical point is this: that number is not fixed; it varies from screen to screen and venue to venue.

Picture the same ten-second play. On a screen in a busy airport corridor that play carries a high multiplier, because many people pass in front of it at that moment. The same creative on a screen in a low-traffic office lobby has a much lower multiplier. The multiplier — the “value” of a play — depends entirely on that screen’s audience volume at that moment.

This is why the impression multiplier can also be a fractional value. In OpenRTB 2.6, the imp.qty.multiplier field is designed for exactly this and supports fractional values such as 14.2 or 0.32. A multiplier greater than one means each play reached more than one person on average; a multiplier below one means each play reached fewer than one person on average. How the multiplier flows through to budget becomes clearer alongside CPM (cost per thousand impressions), which we cover in the DOOH CPM and pricing article.

Worked example: same play, three different venues

To make the multiplier logic concrete, consider a simple example. Suppose a brand buys 100 plays a day. The same number of plays produces very different impression totals depending on the venue.

Venue	Daily plays	Impression multiplier (~)	Total impressions (~)
Airport corridor	100	25.0	2,500
Roadside screen	100	8.0	800
Office lobby	100	0.4	40

In all three rows the ad plays the same number of times, yet the reached audience ranges from 40 to 2,500. The multiplier values in the table are approximate and for illustration only; real values are set by the measurement method and the screen’s traffic. This example shows why “how many plays did I buy” is not meaningful on its own, and the real question is “how many impressions, and at what quality.”

Modeled or measured: how does counting change?

The reliability of an impression count depends on how the multiplier is produced. In the modeled approach, the multiplier is a statistical estimate derived from mobile-location and panel data; it rests on “how many people pass this screen on an average day.” This method scales broadly but cannot know the actual crowd at the precise moment of a given play.

In the measured approach, the multiplier is derived from the headcount actually observed during that play by a screen-side camera and computer vision. This ties counting to the real moment: an empty street at midnight produces a genuinely low multiplier, while a midday crowd produces a high one. The measured approach counts the person looking at the screen by detecting a face only, without recognizing identity; the output is anonymous and aggregate. The two approaches carry different levels of confidence, and the reported impression count changes accordingly.

Summary

In DOOH, an impression measures reached audience, not the number of plays. A play is converted into billable impressions via a venue-specific impression multiplier; whether modeled or measured, that multiplier is the critical number that determines a campaign’s real value.