The UK's TV audience measurement organisation BARB will begin reporting viewing figures across multiple screens from September.
The announcement is the latest stage of the organisation's Project Dovetail, which is looking at the way the audience has changed and fragmented as viewing habits have changed.
The new figures will include the number of people watching programmes across four screens: TV sets, tablets, PCs and smartphones.
BARB says it will use two data sources to collect the information: software meters that are installed on tablets and PCs in its nationally representative panel of UK homes; and census-level streaming data from viewing apps that have been enabled by their owners to deliver duration-based information to Kantar Media, BARB's chosen contractor.
The organisation says this audited measure of online viewing has been delivered in line with best-in-class, joint industry principles.
The next priority for Project Dovetail, after the launch of the cross-platform programme viewing figures, is to deliver an equivalent measure for viewing online TV commercials to enable the industry to assess the total reach and frequency of advertising campaigns across multiple screens.