Calories: Energy Units

Energy is a quantity that describes the ability to supply heat or perform work. It requires units just like any other quantity, such as mass or volume. There are two common units for energy:

  • joule (abbreviated J): 1 joule is defined as the amount of energy used when a force of 1 newton moves an object 1 meter
  • calorie (with a lowercase “c,” abbreviated cal)
    • 1 calorie is the amount of energy needed to warm 1 g of water by 1 ºC
    • 1 calorie is equal to 4.184 J
  • Calorie (with a capital “C,” abbreviated Cal)
    • 1 Calorie is the amount of energy needed to heat 1 kg of water by 1 °C
    • 1 Calorie is equal to 1000 calories
    • 1 Calorie is also equal to 1 kcal (because using the metric prefix “kilo-“: 1 kcal = 1000 cal)

Although the joule is the proper SI unit for energy, we will use the calorie or the kilocalorie (or Calorie) in this chapter because they are widely used to describe energies of food and on nutrition labels.

Calories and Food

The food in our diet provides the energy our bodies need to function properly. The energy contained in food could be expressed in joules or calories, but the food industry prefers to use the kilocalorie and refers to it as the Calorie (with a capital C). The calorie is a rather small quantity, so it is inconvenient to use the calorie to describe energy content of foods on nutrition labels. For example, a candy bar may provide 120,000 cal of energy, which is much simpler to write as: 120 Cal (nutritional calories).

The average daily energy requirement of an adult is about 2,000–2,500 Calories, which is 2,000,000–2,500,000 calories (with a lowercase c). If we expend the same amount of energy that our food provides, our body weight remains stable. If we ingest more Calories from food than we expend, however, our bodies store the extra energy in high-energy-density compounds, such as fat, and we gain weight. On the other hand, if we expend more energy than we ingest, we lose weight. Other factors affect our weight as well—genetic, metabolic, behavioral, environmental, cultural factors—but dietary habits are among the most important.

The three main macronutrients in food are proteins, carbohydrates, and fats. All carbohydrates supply approximately 4 Cal/g. Proteins, the building blocks of structural tissues like muscle and skin, also supply about 4 Cal/g. Other important energy sources in our diet are fats. Fats provide even more energy per gram, about 9 Cal/g. Nutritional labels on food packages show the caloric content of one serving of the food, as well as the breakdown into Calories from each of the three macronutrients.

Two pictures are shown and labeled a and b. Picture a shows a close-up of a bowl of macaroni and cheese. Picture b is a food label that contains highlighted information in a table format. The top of the label reads “Sample label for macaroni and cheese.” Below this are the words “Nutrition facts.” Below this are two lines of highlighted text that read “Serving size one cup (228 g)” and “Servings per container 2.” A label to the left of these lines reads “Start here” and a right-facing arrow is beside these words. Below this are the words “check calories” which lie to the left of the phrases “Amount per serving” which is above the words “Calories 250” and “Calories from fat 210.” The next segment of the label is highlighted and contains five phrases “Total fat 12 g,” “Saturated fat 3 g,” “Trans fat 3 g,” “Cholesterol 30 m g,” and “Sodium 470 m g.” The phrase “Limit these nutrients” lies to the left of these five phrases. The phrase below these is “Total carbohydrates 31 g” and is followed by a highlighted phrase, “Dietary fiber 0 g.” Below this are the phrases “Sugars 5 g” and “Proteins 5 g.” Below this is a highlighted portion containing the phrases “Vitamin A,” “Vitamin C,” “Calcium,” and “Iron.” A label to the left of these terms states “Get enough of these nutrients.” The bottom of the label is labeled “Footnote” and reads “Percent daily values are based on a 2,000 calorie diet. Your daily values may be higher or lower depending on your calorie needs.” Each of the highlighted terms in the table are in line with a percentage value to the right of the table. A note on the outer right of the table states “Quick guide to % DV”, “5% or less is low” and “20% or more is high. The daily value for total fat is 18%, for saturated fat is 15%, for cholesterol is 10%, for sodium is 20%, for total carbohydrates is 10%, for dietary fiber is 0%, for vitamin A is 4%, for vitamin C is 2%, for calcium is 20%, and for iron is 4%.” At the very bottom is a table that indicates calories at 2,000 and 2,500. For total fat the table indicates less than 65 g for 2,000 calories and 80 g from 2,500 calories. For saturated fat the table indicates less than 20 g for 2,000 calories and 25 g for 2,500 calories. For cholesterol the table indicates less than 300 m g for 2,000 calories and 300 m g for 2,500 calories. For sodium the table indicates less than 2,400 m g for 2,000 calories and 2,400 m g for 2,500 calories. For total carbohydrate the table indicates 300 g for 2,000 calories and 375 g for 2,500 calories. For dietary fiber the table indicates 25 g for 2,000 calories and 30 g for 2,500 calories.
(a) Macaroni and cheese contain energy in the form of the macronutrients in the food. (b) The food’s nutritional information is shown on the package label. In the US, the energy content is given in Calories (per serving); the rest of the world usually uses kilojoules. (credit a: modification of work by “Rex Roof”/Flickr)

The total Calories per serving of a food is listed near the top of the nutrition label. For the example shown in part (b) of the figure above, the total energy per 228-g portion is calculated by:

(5 g protein × 4 Cal/g) + (31 g carb × 4 Cal/g) + (12 g fat × 9 Cal/g) = 252 Cal

Calculating Total Calories Example

An 8 oz serving of whole milk has 8.0 g of fat, 8.0 g of protein, and 13 g of carbohydrates. How many Calories does it contain?


The caloric content of fat is 9 Cal/g. The caloric content of both proteins and carbohydrates is 4 Cal/g. To calculate the total calories in one serving, multiply the correct caloric content value by the grams of each food type:

(8.0 g fat × 9 Cal/g) + (8.0 g protein × 4 Cal/g) + (13 g carbs × 4 Cal/g) = 156 Cal

The final answer is 156 Cal.

Measuring Nutritional Calories

So, you can use food labels to count your Calories. But where do the values come from? And how accurate are they? The caloric content of foods can be determined by using bomb calorimetry; that is, by burning the food and measuring the energy it contains. A sample of food is weighed, mixed in a blender, freeze-dried, ground into powder, and formed into a pellet. The pellet is burned inside a bomb calorimeter, and the measured temperature change is converted into energy per gram of food.

Today, the caloric content on food labels is derived using a method called the Atwater system that uses the average caloric content of the different chemical constituents of food, protein, carbohydrate, and fats. The average amounts are those given in the equation and are derived from the various results given by bomb calorimetry of whole foods. The carbohydrate amount is discounted a certain amount for the fiber content, which is indigestible carbohydrate. To determine the energy content of a food, the quantities of carbohydrate, protein, and fat are each multiplied by the average Calories per gram for each and the products summed to obtain the total energy.

Click on this link to access the US Department of Agriculture (USDA) National Nutrient Database, containing nutritional information on over 8000 foods.


This page is based on “Chemistry 2e” by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhDOpenstax which is licensed under CC BY 4.0. Access for free at

This page is based on “The Basics of General, Organic, and Biological Chemistry” by David W Ball, John W Hill, Rhonda J ScottSaylor which is licensed under CC BY-NC-SA 4.0. Access for free at


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Chemistry of Food and Cooking Copyright © 2022 by Jessica Wittman is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.