Measure viewability, not impressions

The industry counts impressions. An impression occurs when an ad is served, regardless of whether a human could see it. That is the core of a misunderstanding that costs advertisers substantial sums every year.

What viewable officially means

The common industry definition requires, for a display ad, that 50 per cent of the area was in the visible viewport for at least one second. For video it is 50 per cent of the area for two seconds.

That threshold is low. An ad that scrolls half-visible past the bottom edge for one second counts as viewable.

What actually happens

Typical viewability rates in practice:

At 45 per cent viewability you pay for 1,000 impressions and get 450 chances. The apparently cheap £2 cost per thousand is an effective £4.44.

The sum you should do

Never compare offers on cost per thousand; compare on cost per viewable contact.

1. Offer A: £2.00 CPM at 45 per cent viewability gives £4.44 effective.
2. Offer B: £3.50 CPM at 82 per cent viewability gives £4.27 effective.

Offer B is cheaper and delivers a better environment as well. In practice that calculation regularly reverses the ranking.

How to measure

Do not rely on the seller's figures. An independent verification service costs one to three per cent of budget and returns three numbers that together give a picture: viewability rate, share of invalid traffic, and environment suitability.

Agree a make-good as well: if measured viewability falls below the promised figure, the sales house delivers additional inventory free. That is common practice and is rarely offered unprompted.

The next step: attention

Viewability says whether an ad was visible. Attention measurement tries to capture whether anyone actually looked, using eye tracking in test panels, dwell time in view and interaction signals.

Early findings from these methods matter more for planning than the viewability debate. Large formats deliver disproportionately more attention seconds, and attention seconds correlate far better with brand effect than impressions do. A format that costs twice as much and delivers four times the attention seconds is the cheaper choice.