Solved: What is a Calorie?

You'll benefit from this if you already have a solid understanding of basic nutrition and why it's important.

For everyone else, this article will help build your foundational nutritional knowledge and confidence as you move forward.

If you just started learning how diets work, try skimming this article for beginners; it's still a useful refresher.

Brunette Female scientist smiling with arms crossed

Summary: What is a Calorie? (TL;DR)

  • Calories are a unit of energy - The energy in food is determined by adding up its macronutrients.
  • Protein, carbs, and fat supply our energy - Protein and carbs contain 4 calories per gram, fats 9 per gram.
  • Alcohol isn't a nutrient - It interferes with the growth, maintenance, and repair of the body.
  • Calories are calculated by burning things - Scientists use bomb calorimeters to provide caloric estimates.
  • You can count calories in your homemade foods - Add up the total number of macronutrients from each food.
  • Energy-dense foods contain more calories per gram - Raisins are denser than grapes.

What is a Calorie in Food?

Calories are a measure of the energy that we get from food.

Outside of the United States, calories are expressed in kcalories or kilocalories. What Americans know as "calories" are 1000 tiny kilocalories.

(We're one of the only three countries left who haven't adopted the metric system.)

To make life easier, I will also shorten kilocalories into "calories".

You'll recall that there are six major classes of nutrients:

  1. Water
  2. Minerals
  3. Carbohydrates
  4. Fats
  5. Proteins
  6. Vitamins

Protein, carbohydrates, and fat are examples of the nutrients that supply energy. Each of these is what we call a macronutrient. They are the nutrients we count when we're talking calories.

What each macronutrient has in common is the carbon atom. Vitamins also contain carbon but, don't provide energy. Vitamins help our body release energy from carbs protein and fats.

What about alcohol?

Alcohol does contain calories but, it is not considered a nutrient.¹

"Unlike the essential nutrients, alcohol does not sustain life. In fact, it interferes with the growth, maintenance, and repair of the body. Its only common characteristic with nutrients is that it yields energy (7 kcalories per gram) when metabolized in the body."
(Rolfes, Whitney, & Pinna, 2015)

Now that you know what they are, let's talk about how to count calories.

How are Calories Calculated?

Calories are calculated by measuring the amount of each macronutrient contained in foods.

The amount of energy per gram varies by macronutrient:

  • Carbohydrates - 4 calories per gram.
  • Protein - 4 calories per gram.
  • Fats - 9 calories per gram.
  • Alcohol - 7 calories per gram.

Scientists use a tool known as a bomb calorimeter to burn foods and release the energy they contain as heat. This heat energy is then converted into kilocalories.

First, they burn a variety of the same food. Then, scientists determine the amount of energy available in a serving of each.

Not all apples have the same number of calories.

You can get an estimate for the average apple via reference charts or a resource like Food Data Central.

- There is a difference between the number of calories in food and what you absorb.

Bomb calorimeters are so efficient, they convert almost all the energy found in food. Like your car, your body loses some of the energy it creates when breaking down fuel. Researchers try to correct for this conversion with reasonable estimates.

Researchers provide tables containing estimates of the usable energy that food bears.

The potential inaccuracy is why some say that counting calories "doesn't work". It can, countless people have lost weight using this method. You can get some reasonable estimates if you choose to count calories.

How to Measure Calories in Food at Home

Let's look at an example of the calories contained in a classic American staple, a PB&J sandwich.

In this case, we will use three items from Food Data Central:

Examples of calories

Food: Protein (g): Carbohydrates (g): Fats (g):
(2) Slices of wheat bread 4.0 28.0 2.0
(2) Tablespoons of peanut butter (32g) 7.1 7.14 16.44
Tablespoon of grape jelly (20g) 0.0 13 0.0

Here's how you can find the calories available in your food:

1. Count your carbs

  • Add up the total number of carbohydrates in your bread, peanut butter, and jelly. That comes out to around 48.14 grams.
  • Multiply the number of grams of carbohydrate available by 4.

Total calories from carbohydrates - 192.56

2. Produce your protein

  • Add up the sum of protein in your bread, peanut butter, and jelly. That comes out to around 11.1 grams.
  • Multiply the number of grams of protein available by 4.

Total calories from protein - 44.4

3. Figure out your fats

  • Add up the number of fats in your bread, peanut butter, and jelly. That amount is 18.44 grams.
  • Multiply the number of grams of fat available by 9.

Total calories from fat - 165.96

4. Add up all the calories to get the total energy available

Carbs (192.56) + Protein (44.4) + Fat (165.96)= 402.92 Total calories in your PB&J sandwich.

What is the Difference Between Nutrient Dense and Calorie Dense Foods?

Nutrient dense foods are foods that contain more nutrients per gram.

When foods have more calories per gram, we call them energy-dense. When a food has more nutrients per gram we call it nutrient-dense.

The same basic foods might vary greatly calorically in a different form.

For Example:

1 cup of raisins (145 grams, not packed)

  • 434 calories
  • 0.7 grams of fat
  • 115 grams of carbohydrate
  • 4.5 grams of protein
  • 22.4 grams of water

1 cup of seedless grapes (151 grams)

  • 104 calories
  • 0.2 grams of fat
  • 27.3 grams of carbohydrate
  • 1.1 grams of protein
  • 122 grams of water

The biggest difference between these two foods is the amount of water in each.

By eating grapes, you could eat 4x the number of cups and still consume fewer calories overall.

You can see how choosing nutrient-dense foods leave you feeling fuller for the same number of calories.

While some approaches boast weight loss "without counting calories", they're typically diets that involve eating less calorically dense (low calorie) foods.


  1. Rolfes, S. R., Whitney, E. N., & Pinna, K. (2015). An Overview of Nutrition. In Understanding Normal and Clinical Nutrition (Tenth ed., p. 9). Stamford, CT: Cengage Learning.

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Steven Mack is founder and a Certified Strength and Conditioning Specialist at the private training studio, Simple Solutions Fitness. He consults for Stronger by Science, a leader in fitness research dissemination, and is a former Mizzou football walk-on. Steven dedicates his professional life to helping people through his writing, speaking, and role as a personal trainer.