Co-viewing refers to situations where multiple people watch YouTube together on the same screen, most commonly on connected TVs (CTV). When this happens, a single ad impression may actually be seen by more than one person, which can increase the true reach of a campaign.
Google estimates co-viewing using a combination of connected TV panel data, large-scale surveys, and statistical modeling. This helps measure how many people are likely watching together, based on factors such as time of day, day of the week, country, and audience demographics.
For advertisers, co-viewing metrics provide a more accurate understanding of reach and impressions on TV screens. This makes it easier to compare YouTube campaigns with traditional TV or other connected TV advertising and to make better budgeting and planning decisions.