When you go into a convenience store, you might find hot dogs spinning on a roller grill or an egg salad sandwich in a cold case. Just how many calories do those items have?

Ignorance may be bliss, but not for the Food and Drug Administration. The agency published new rules today on posting calorie counts that will take effect in a year. They will apply to such food providers as chain restaurants, vending machines with more than 20 locations and grocery stores that serve prepared foods. Some places, like Starbucks and New York City, already post this information.

So how does a food-based enterprise actually count a calorie?

Lyle Beckwith, with the National Association of Convenience Stores, isn’t yet sure how to answer that question.

“We’ve not been in this business before,” he says. “I’m assuming there are labs that we’ll have to send various products out for testing. The cost and time to be determined.”

In fact, there are private companies that do this work. James McKnight works at one of them, QC Laboratories in Pennsylvania.

“All they have to do is pretty much send us a sample, you know, via UPS, FedEx, give us a serving size and off we go,” McKnight says.

The FDA estimates these new rules will cost the restaurant industry $85 million over 20 years. QC Laboratories charges $700 per sample for full nutritional information or $150 to $200 if it’s simply using a database of ingredients to calculate the tally.

Most companies use the database method, says Jim Painter, a professor at Eastern Illinois University who studies food science.

To calculate the calories in a slice of pizza, Painter says “you’d have the flour, you have a little bit of sugar, you have the salt, tomato sauce. You have whatever ingredients you put on the top and you’d add up all those, you’d divide it by what a portion would be.”

It’s generally pretty easy, according to Painter, unless the items change frequently or involve a great deal of customization. Because the databases are built on averages, they’re not entirely accurate.

“But at least it gives you a comparison when you’re looking at one food compared to the other,” Painter says.

That way customers can compare that slice of pizza to a hot dog, or the egg salad. Or maybe opt for an apple instead.