By Lisa Baertlein

Wed Sep 12, 2012 10:54am EDT

(Reuters) – McDonald's Corp will begin telling diners next week how many calories are in its Big Macs (550) and french fries (500 for a large size).

The world's No. 1 hamburger chain said on Wednesday it is going to start listing calorie information on menus in some 14,000 U.S. restaurants and drive-thrus – ahead of a national rule that will require larger restaurant chains to make the disclosures.

The state of California and cities like New York already require that calories be clearly listed on menus.

Under the new U.S. healthcare law, restaurants must soon clearly post calories and other nutrition details on menus. The rules target restaurants with 20 or more locations, as well as other retail food outlets. Most major chains have resisted, without legislation and the threat of fines.

McDonald's, under pressure from consumer groups and public health advocates, has also taken steps to make menu items healthier.

It tweaked its popular Happy Meals for children – reducing the french fries portion by more than half and adding apples automatically to every order.

McDonald's was slow to adopt calorie labeling. When the labeling requirements were gaining interest several years ago, a McDonald's nutritionist told a group of health reporters in Los Angeles that the company opposed the rules because they violated customer privacy.

More than two years ago, Panera Bread Co became the first national restaurant chain to voluntarily post calories at company-owned stores. Sandwich chain Subway has used calorie disclosures to position itself as a healthier alternative to rivals like McDonald's and Burger King Worldwide Inc.

McDonald's shares were down 0.3 percent at $90.97 on Wednesday on the New York Stock Exchange.

(Reporting By Lisa Baertlein in Los Angeles; Editing by Jeffrey Benkoe)