American magazine publishers will be able to count iPad subscriptions in their circulation figures.

The Audit Bureau of Circulations in the US has changed its definition of a digital magazine to accommodate Apple's tablet-style devices.

Publishers will be able to design articles and photos for the iPad, which goes on sale on 3 April in the US and soon after in Britain. They can count paid-for digital subscriptions in figures as long as the same editorial and advertising material is included.

Without the rule change, they could only count digital editions that appear exactly the way they do in print.

Magazines need the change because they charge for ads based on the size of the circulation they guarantee to advertisers, known as the rate base.

ABC must approve each software application for the iPad and other devices for the figures to count. It has allowed Conde Nast's Wired magazine to release an app, which will be available for the iPad in June. Conde Nast is also planning iPad versions of GQ, The New Yorker, Vanity Fair and Glamour.

Subscriptions on many e-readers, such as Amazon's Kindle, do not count because those editions do not carry adverts, but the iPad will have a backlit screen that can carry colour advertising alongside articles.

The main problem publishers face is that few consumer magazines or newspapers have succeeded in getting large numbers of readers to pay for access to websites.

About 6,800 people paid $2.99 in December to download the first issue of GQ's iPhone app, which meets the auditors' guidelines. This is against the magazine's overall circulation of just under 900,000, although January downloads more than doubled to about 15,100.