Viewability
Whether a served ad had a chance to be seen: by the MRC and IAB standard, at least 50% of the ad's pixels on screen for at least one second for display and two seconds for video. It removes impressions that loaded below the fold or in a background tab. Viewable is still not viewed: attention research shows many viewable ads get little or no human attention, so viewability is a minimum, not a goal.
Where it's used
Used in programmatic buying, publisher deals and campaign reporting to stop paying for ads nobody could see. Good as a minimum standard in buys, together with attention and outcome measures.
How to calculate it
Cost per thousand viewable impressions (vCPM) = Spend ÷ Viewable impressions × 1,000
Worked example
Illustrative numbersA display buy delivers 1,000,000 impressions at a CPM of 4. Only 55% of them meet the viewability standard.
Spend
1,000,000 impressions ÷ 1,000 × 4 CPM = 4,000
Viewable impressions
1,000,000 impressions × 55% = 550,000
Real cost per thousand viewable
4,000 spend ÷ 550,000 viewable × 1,000 = 7.27
What it tells you
The buy looked like a CPM of 4 but cost 7.27 per thousand ads that could be seen. Compare offers on vCPM, then ask how many of those were watched.