Viewability refers to whether online ads are in the visible part of a user’s screen for a minimum period of time. These measurement standards are designed to provide more transparency to advertisers. A ‘viewable’ banner ad has at least 50% of the ad appear on a user’s screen for at least 1 second. For large format ads, it’s 30% for 1 second. When bidding on display ad placements, all CPM-based bids now use vCPM. This means advertisers don’t pay for an impression unless their ad is actually viewable by the user.