What Are Calories and How Are They Measured?

What Are Calories and How Are They Measured?

What are calories?

If you’re just starting out on your fitness journey, you might know calories as the things you stare at on the back of a cereal box while scrunching your nose in disdain and/or confusion. A measly cup of cereal has how many calories?

But what actually is a calorie? How is it measured? Is it really a reliable measure that’ll help you monitor your diet? Read on.


How a Calorie Is Measured

Food’s main biological role is to fill our needs for energy and nutrients — think vitamins, minerals, protein, carbs, fats, and phytonutrients. Scientists need a way to measure this energy, and they do so in the form of a calorie: By definition, a Calorie (with a capital “C”) we commonly see on the back of nutrition labels is actually a kilo-calorie, meaning it’s made up of 1,000 gram calories.

Each Calorie is capable of heating up one kilogram of water by one degree Celsius.

In the past, food was burned in a bomb calorimeter, and the energy released was used to heat the surrounding water. Depending on how hot the water got, scientists would determine the food’s calorie value.

Today, the process is a little less exciting. Calorie counts are guesstimated by summing up the calories from macronutrients in food based on these rules of thumb:

  • Carbohydrate = 4 Calories per gram
  • Fat = 9 Calories per gram
  • Protein = 4 Calories per gram
  • Alcohol = 7 Calories per gram, but alcohol is considered to be a source of empty calories, meaning it has no real nutritional value

what are calories- woman in fridge

How Calories Lead to Weight Gain

Calories became a popular way to help explain weight maintenance in a sleek concept called energy balance: The premise is that if the amount of energy you eat (calories in) is equal to the amount of energy you expend (calories out), then your weight should remain the same. If you’re consuming more calories than you’re expending, this will lead to weight gain.

This concept is endorsed by expert organizations like the Centers for Disease Control; many weight loss and diet programs encourage people to eat less and move more, in order to create enough calories out for weight loss.

It’s a useful general rule, though the reality is actually a little more complicated than that.

The types of calories you’re consuming are important, too — you can’t hit your numbers solely via cupcakes and expect to be healthy (more on this below). And several factors like age, gender, and activity level can also affect your individual caloric needs.


How to Count Calories

If you’re looking to count your calories, online tools like this one can help give you a general idea of where you’re at.

But since all calories are not equal, and each person’s needs are different, you should consult your doctor or dietitian to determine your specific calorie goals.

nutrition label- what are calories

Are Calorie Counts on Food Labels Accurate?

Not too long ago, an article from the New York Times quoted nutrition experts who stated that labels on foods high in proteins and fiber can be overestimated by as much as 25 percent.

The reason? The human body uses more energy to digest certain foods, such as nuts and meats, than it does to break down highly processed carbohydrates, such as cookies and bagels.

Robert Dunn, a biologist at North Carolina State University, recently wrote in Scientific American that “digestion is an intricate process,” and that calorie numbers on labels “are inaccurate because they are based on a system of averages that ignore the complexities of digestion.”

Dunn goes on to say that our ability to extract calories varies depending on how long our intestines are, what gut bacteria we carry, and even how well we cook our food.


Is a Calorie a Calorie?

At this point, you may wonder if calories even matter. They do.

The point is that, while you don’t need to be nit-picky about counting every last calorie — there are too many other factors in play — you should still be generally aware of how much food you’re putting into your body. And calorie counting is one way to drive this awareness.

But while the overall quantity of calories consumed is important for weight loss, the quality of your calories is equally important when creating a healthful diet.

What does all of this boil down to? Food quality matters.

Experts consistently recommend that you reach for minimally processed foods like fruits, vegetables, whole grains, and healthy sources for fats and protein.

It may seem like common sense, but it’s also science. These foods are nutrient-dense, meaning they contain many different vitamins and minerals for relatively few calories. This is especially helpful for weight loss.

Ultimately, meeting your weight-loss goals involves understanding how much food you need to eat (calorie quantity) and which foods are best for your health (calorie quality).

Trinh Le


Trinh is a registered dietitian by day, blogger at Fearless Food by night. She loves helping folks develop a better relationship with food, which includes lots of cooking, eating and learning about nutrition. When she’s not snapping mouthwatering shots of (mostly) healthy food, you can find Trinh HIIT-ing it at her local gym. Follow her on Twitter.