Stuck in a nearly decade-long decline in print advertising, magazines are adopting a new way of measuring themselves that casts aside the longtime practice of tracking ad pages. On Sept. 29, the MPA - the Association of Magazine Media, a trade group representing most publishers, will introduce Magazine Media 360, which combines magazines' print and tablet audience, unique visitors to their web and mobile sites and unique video views. It intends to publish this number monthly, along with a Social Media Report tracking magazines' social-media presence.

"We're simply telling a story of consumer demand," said Mary Berner, president-CEO of MPA. "It's stunning how it redefines magazine media."

The move marks a significant change for an industry that's trying to shift the conversation away from its weaknesses.

Newsstand sales are down 12% through the first six months of 2014 and paid subscriptions are off 2%. With few exceptions, advertising comprises the bulk of magazines' revenue, but many marketers are redirecting their print-ad budgets to digital. Although magazines' digital-ad sales have grown sharply, they amount to a fraction of print revenue.

The August 360 score, which compiles numbers from all MPA publishers, is nearly 1.5 billion, a 9.8% increase over last August, driven by mobile and video. "It opens the eyes of marketers and agencies who always try to look at a magazine in a vertical way," said Michael Clinton, a Hearst Magazines exec and MPA chairman.

For decades, magazines have evaluated themselves partly on the number of print advertising pages in each of their issues. The idea was that the more ad pages a magazine runs, the more money it receives from advertisers. But the industry has not posted an annual increase in ad pages since 2005, according to the Publishers Information Bureau. "If the ad page numbers were good, they wouldn't be doing this," said Steve Cohn, editor-in-chief of Media Industry Newsletter.

Ms. Berner said the MPA isn't trying to hide anything. "Paging metrics in a world where 80% of magazine media advertisers invest in other platforms in addition to print reflect only one format and simply aren't comprehensive and therefore not accurate."

The magazine industry is the only medium that has measured itself using advertising, said Mr. Clinton. TV looks at ratings; digital-only companies track unique visitors. "We're the only orange in a basket of apples."

Robin Steinberg, exec VP-director of publishing investment and activation at MediaVest, commended the magazine industry for showing the scale of its brands. "But it's also important to understand the advertising marketplace demand and vitality for the independent channels as it aids to inform strategic decisions and outcomes," she said.