The Washington Post will phase in a paid online subscription model for Web content starting June 12. (Bill O'Leary/The Washington Post)

The Washington Post will phase in a paid online subscription model for Web content starting June 12, charging some readers $9.99 a month for access to more than 20 articles a month on desktop and mobile devices.

For $14.99 a month, readers can get a premium package that includes access to all of The Post’s custom apps, which make it easier to see material on gadgets such as iPads and iPhones.

The plan will not affect substantial numbers of The Post’s readers, who will continue to have unlimited free access online. Readers who subscribe to home delivery of the print edition, including those who receive only the Sunday edition, will be asked to register for the Web site, but they won’t pay anything extra.

In addition, students, teachers, school administrators, government employees and military personnel who sign on from their schools or workplaces will not have to pay for online access. However, if they log on from home, they will have to pay for unlimited access to the paper’s Web site.

In addition, the newspaper said in an announcement, “visitors who come to The Post through search engines or shared links will still be able to access the linked page regardless of the number of articles they have previously viewed.” That includes readers who view an article through a link on Facebook or Twitter.

Access to The Post’s homepage, section front pages, videos and classified advertising will not be limited for any readers. Photo galleries that encourage readers to flip through several pages of content will count only once against the 20-article limit. But individual entries on blogs will each count toward the cap.

The newspaper’s publisher, Katharine Weymouth, said the company will “listen to reader feedback and modify our model accordingly. There is going to be a great deal of experimentation ahead to strike the right balance between ensuring access to critical news and information and building a sustainable business.”

The Post has been cautious about following the lead of newspapers that have instituted what is commonly known as a “paywall.” Some senior executives have worried that the paper would lose too many readers, especially from the national audience outside the Washington area. Others, however, have argued that the paper needs to build new sources of revenue and that readers are growing used to the idea of paying for newspaper content online.