How Are Calories in Food Measured?

How do food manufacturers measure the calories in food? How do they know what to put on nutrition labels? Everyday Einstein discusses several methods of calorie measurement (and explains why they're all problematic). 

Lee Falin, PhD
4-minute read
Episode #120

chocolateAt some point in your life, you’ve probably been a calorie counter, if only for a moment.

Have you ever looked at a food nutrition label and wondered what exactly a calorie is? Everybody knows that foods high in calories are supposed to be worse for you, right? Or is that only when you are dieting?

Let’s look at some of the science behind calories to see what’s what.


What Is a Calorie?

In scientific terms, a calorie is a unit of energy, just like a meter is a unit of distance, or a gram is a unit of mass. One calorie is the amount of energy you need to raise 1 gram of water 10 Celsius.

So if you had an 8-ounce cup of water (which would be about 224 grams of water), and you wanted to make it 50 warmer, you would need 224 x 5 calories, or 1,120 calories.

Now one thing you should know is that in the United States, when a food label says that a candy bar has 300 Calories, those Calories are spelled with a capital C. This is sometimes called a “nutritionist calorie,” but in reality it means a kilocalorie, or 1,000 calories.

If you look at a food label in Europe, unless it’s imported food from the U.S., it will show the number in kilocalories.

For the conscientious calorie counter, keep in mind that neither of these caloric measurements necessarily tell you how many calories are in the entire candy bar. In the U.S., the measurement is the amount of kilocalories per serving (which varies from package to package), while in Europe it is how many kilocalories per 100 grams. European labels also usually have a measurement in kJ or kilojoules, which is just another unit used to measure energy (1 calorie is about 4.2 joules).

How Are Calories Measured?

So now that you know what a calorie is, let’s talk about how a calorie is measured.

To measure the calories in a random candy bar, all you have to do is dehydrate it, then burn it. While it’s burning, just measure how much heat energy it gives off, and that will tell you how many calories it has—super easy!

While that might seem crazy, that’s really how it’s done. Remember what we said about a calorie being the amount of energy required to raise 1 gram of water 10 Celsius? To measure the heat given off when you burn your candy bar, first place it inside a metal can, then place that metal can inside of an insulated container of water. Then stick a thermometer in the water, seal the whole thing up, and you have what is called a calorimeter.

As the candy bar burns, the heat goes through the metal container, and heats up the water. If you know how much water you have, you can just measure how high the temperature gets, and use that to calculate the energy.

Of course, real life calorimeters are slightly more complicated than that, but this gives you the basic idea.


About the Author

Lee Falin, PhD

Dr. Lee Falin earned a B.S. in Computer Science from the University of Illinois, then went on to obtain a Ph.D. in Genetics, Bioinformatics, and Computational Biology from Virginia Tech.