WASHINGTON — The Food and Drug Administration announced sweeping rules on Tuesday that will require chain restaurants, movie theaters and pizza parlors across the country to post calorie counts on their menus. Health experts said the new requirements would help combat the country’s obesity epidemic by showing Americans just how many calories lurk in their favorite foods.

The rules will have broad implications for public health. As much as a third of the calories that Americans consume come from outside the home, and many health experts believe that increasingly large portion sizes and unhealthy ingredients have been significant contributors to obesity in the United States.

“This is one of the most important public health nutrition policies ever to be passed nationally,” said Margo Wootan, director of nutrition policy at the Center for Science in the Public Interest. “Right now, you are totally guessing at what you are getting. This rule will change that.”

The rules are far broader than consumer health advocates had expected, covering food in vending machines and amusement parks, as well as certain prepared foods in supermarkets. They apply to food establishments with 20 or more outlets, including fast-food chains like KFC and Subway and sit-down restaurants like Applebee’s and The Cheesecake Factory.