MINNEAPOLIS (WCCO) — A McDonald’s Big Mac has 540 calories and a Panera Bread Frontega Chicken sandwich has 740. By the end of 2016, the FDA will require larger chain restaurants, fast-food places and some business that sell food to post this calorie information.

So, Chris from Madison asks: How are calories measured in food? Good Question.

Calories are a measure of energy. Technically, one calorie is the amount of energy it takes to raise one gram of water one degree Celcius. Food is measured in kilocalories, so it’s actually 1000 calories for every 1 calorie listed.

“There are actually a couple of ways this can be done,” says Holly Willis, director of dietetics and nutrition at St. Catherine University.

First, the food can be sent to a laboratory to be analyzed through a number of mechanisms.

“They would ask for a full sample of, say, chicken chipotle sandwich,” says Willis. “They’re going to mix it up into a pulverized version of that, take a small sample out and then they’ll take an analysis.”

Sometimes, scientists will burn the food in a bomb calorimeter to measure how much energy the fats, proteins and carbohydrates give off. This can be calculated by measuring the temperature change to determine how much energy is in the food.

“The easier way would be to do a manual calculation using food databases,” says Willis.

The U.S. Department of Agriculture maintains a public database where nutrients and calories can be calculated. With this system, you would look up every single ingredient in a food and add up the nutrients that contain energy. Proteins and carbohydrates are 4 kcal/gram and fats are 9 kcal/gram.

The FDA requires restaurants and businesses to take “reasonable steps” to make sure their calorie counts are what they say. A study in the Journal of the American Medical Association found they were accurate overall, but did find 20% of foods went over by 100 calories.

“If you have someone with a heavy hand with the guacamole or it’s the end of the evening and you’re running short on shredded chicken, there’s going to be some variability based on food preparations,” says Willis.

Heather Brown