The only true consensus around the standard for viewability is that it’s a better metric than only saying whether an ad was served or not.

The current Media Ratings Council definition of a “viewable” ad requires static creative to be at least 50% visible for one second, and videos to be at least 50% visible for two seconds.

But everything from whether publishers should only charge for “viewable” ads to the value of viewability itself is still up for debate.