Viewability: You Paid For It, But Was It On Screen?

The short version

An impression means the ad was served. It does not mean anyone could see it.

Viewability is the share of served impressions that were actually on screen, for long enough to have a chance of being noticed.

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Viewability = (viewable impressions / served impressions) x 100

The standard thresholds are conventions rather than laws of nature: broadly, a portion of the ad on screen for a short duration, with video usually requiring longer than display.

Why it exists

Ads get served into places nobody looks. Below the fold on a page the user never scrolls. In a background tab. In an inventory slot that loaded and was scrolled past instantly.

All of those count as impressions, and all of them cost the same as an impression someone saw.

The practical consequence is that your effective CPM is higher than your reported CPM. If viewability is 60%, you paid for ten impressions and got six chances at attention. Your real cost per viewable impression is roughly two-thirds higher than the number in your report.

Where it matters and where it does not

Matters most: display and programmatic, where inventory quality varies enormously and low-viewability placements are common and cheap.

Matters less: major social feed placements, where the format and delivery mechanics make most served impressions viewable by construction. Chasing viewability here optimizes something already near its ceiling.

Matters for video in a specific way: a video impression that was never on screen also never had its hook seen, which corrupts every creative metric downstream of it.

Viewable is not seen

This is the limit worth holding onto.

Viewability is a technical measure of whether pixels were rendered in the viewport for a duration. It says nothing about whether a human looked, understood, or cared.

An ad can be fully viewable and completely ignored. Viewability sets a ceiling on attention; it does not measure it.

So high viewability with poor performance is not a contradiction. It means the ad was seeable and did not work, which points at creative rather than at inventory.

What to do with it

Use it to compare inventory sources, not creatives. It is a property of where the ad ran.
Recalculate your effective CPM on viewable impressions before comparing channels.
Investigate very low viewability as a placement quality problem, and consider excluding the sources responsible.
Do not chase 100%. The most viewable inventory is also the most expensive, and the trade is not always worth it.

Practical rule

Treat viewability as a check on what you actually bought rather than as a target. Recompute your cost on viewable impressions, and if a channel looks cheap only before that adjustment, it was not cheap.