Comcast has already reserved the right to cut off subscribers who use too much bandwidth each month, without specifying exactly what constitutes excessive use.
"We've listened to feedback from our customers who asked that we provide a specific threshold for data usage and this would help them understand the amount of usage that would qualify as excessive," the company said in a statement on its Web site.
Customers who go over the limit are contacted by the company and asked to curb their usage.
"We know from experience the vast majority of customers we ask to curb usage do so voluntarily," the company said.
Comcast floated the idea of a 250 gigabyte cap in May and mentioned then that it might charge users $15 for every 10 gigabytes they go over, but the overage fee was missing in Thursday's announcement.
Curbing the top users is necessary to keep the network fast and responsive for other users, Comcast has said.
Comcast stressed that the bandwidth cap is far above the median monthly usage of its customers, which 2 to 3 gigabytes.
Very few subscribers use more than 250 gigabytes, it said. A user could download 125 standard-definition movies, about four per day, before hitting the limit.
The cap is also above those of some other ISPs. Cox Communications' monthly caps vary from 5 gigabytes to 75 gigabytes depending the subscriber's plan. Time Warner Cable Inc. is testing caps between 5 gigabytes and 40 gigabytes in one market. Frontier Communications Co., a phone company, plans to start charging extra for use of more than 5 gigabytes per month.