Tony Cenicola/The New York Times Movie popcorn contained an alarming amount of fat, salt and calories.

Sitting in a dark movie theater with your friends and a tub of buttery popcorn sounds like a perfect way to spend a Saturday night – and it could be, if you are willing to share your popcorn with the entire row of moviegoers around you.

A review of the nutritional contents of movie-theater popcorn reveals an alarming amount of fat, salt and calories in even the smallest sizes. The study, from the Center for Science in the Public Interest, looked at popcorn from three movie theater chains and detailed the contents of all portions offered.

A large tub of popcorn at Regal Cinemas, for example, holds 20 cups of popcorn and has 1,200 calories, 980 milligrams of sodium and 60 grams of saturated fat. Adding just a tablespoon of butter adds 130 calories. And do not forget that it comes with free refills.

Not so hungry? The medium size popcorn, which comes in a bag, contains the same amount as the large. And even the small, at 11 cups, delivers 670 calories, 550 milligrams of sodium and 24 grams of saturated fat.

The findings may surprise those who choose popcorn at the concession stand because they believe it is a relatively healthy snack. In fact, plain air-popped popcorn is low in calories and free of saturated fat. Movie theater popcorn, however, is popped in oil — often coconut oil, which is 90 percent saturated fat. Add salt to the enormous portions, and your once-healthy snack turns into a health offender.

“The issue here is quantity,” said Marion Nestle, a professor of nutrition at New York University. “One of those large tubs is three-fourths of a day’s calories.”

Even the “healthiest” sample of popcorn tested, a small size from AMC containing 6 cups of popcorn, had 370 calories, enough to justify sharing with a friend and eating one kernel at a time to make it last the length of a movie.

“If you were eating just a cup or two, it wouldn’t matter nearly as much,” Dr. Nestle said.

The third movie theater chain tested, Cinemark, fares slightly better than its competitors because it pops its popcorn in nonhydrogenated canola oil instead of coconut oil. Cinemark’s large tub contains 17 cups of popcorn with 910 calories and 4 grams of saturated fat, as well as a whopping 1,500 milligrams of sodium — nearly enough for the entire day.

One way to make your popcorn healthier? Ask the theater to pop a portion without salt. Two of the movie theaters that had their popcorn tested said they would oblige such a request.

The best way to make your movie snack healthier, however, would be to skip the popcorn — and the concession stand — altogether.

“You could share a tub of popcorn with 10 friends,” Dr. Nestle said. “Or, what a concept, watch the movie without eating anything.”