It’s an unspoken rule that when you go to the movie theater, you buy popcorn. But what size? Why not just get the large? It’s only $1 more than the small, and it’ll last you throughout the two-hour movie.

But would you make the same choice if you were told the larger bucket had nearly 800 more calories than the smaller one?

Consumers now will know exactly what they’re getting, thanks to a new rule from the U.S. Food and Drug Administration that requires nearly all businesses that sell food to display a calorie count. The regulation is in response to rising rates of obesity, and aims to standardize what the agency called a patchwork of menu-labeling laws across the country so consumers get consistent, science-based information.

The rule also requires businesses to have other nutritional information — such as the amount of fiber, sodium or sugar in the food — available upon request.

The FDA cited studies showing that when restaurants have clear, nutritional information on their menus, it can help reduce caloric intake and possibly make a difference in obesity rates. But, not all of the research agrees. Some studies suggest people don’t always choose the healthier option, even with the calorie counts in front of them. In fact, many don’t even notice it.

This rule might not have a positive effect on regular, fast-food goers, but it may change behavior in those who like to spend their Saturday night occasionally going to dinner and a movie.

After dining at your favorite Italian restaurant, buying your movie ticket and debating what size popcorn to get, you might make a new decision after seeing the large popcorn is 1,030 calories — without butter.