U.S. researchers say they've estimated the amount of data stored worldwide by 2007 at 295 exabytes -- equivalent to 1.2 billion average computer hard drives.

The study, published in the journal Science, calculated the figure by estimating the amount of data held on 60 analog and digital technologies from 1986 to 2007, the BBC reported Friday.

The researchers considered everything from computer hard drives to obsolete floppy discs, from X-ray films to microchips on credit cards to DVDs.

"If we were to take all that information and store it in books, we could cover the entire area of the United States or China in three layers of books," Martin Hilbert of the University of Southern California said.

Computer storage has traditionally been measured in kilobytes, then megabytes and now usually gigabytes.

After that comes terabytes, petabytes, then exabytes. One exabyte is a billion gigabytes.

The survey covered a period known as the "information revolution" as human societies transitioned to a digital age.

In 2000, it found, 75 percent of stored information was in an analogue format such as video cassettes, but that by 2007, 94 percent of it was digital.