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What is the Difference Between Calories and Kj?

What is the difference between calories, kilocalories and kj? These are all units of measurement for energy. These measurements are commonly used to tell us how much energy is in food. Keep reading to understand the technical differences and discover why so many people, and companies, get these confused.

plate, knife and fork

What is a Calorie?

A calorie is a unit of measurement. It is used to represent the energy in food and drinks. It is defined as the amount of energy needed to raise the temperature of one gram of water by one degree Celsius. The calorie is also used to quantify the energy our bodies can obtain from what we eat and drink and the amount of energy we use through physical activity.

How are Calories Calculated?

This blog is going to get a bit technical; sorry in advance. Calories in food and drinks are measured based on the carbohydrates, protein, and fats they contain. A calorie is often abbreviated to "cal".

What is the Difference Between Calories and Kj

Calories and kilocalories (kcal) are often used interchangeably and are a way to measure energy. One calorie (cal) is equivalent to 1/1000th of a kilocalorie (kcal). Both measure the energy content in food, with 1 kcal equal to approximately 4.184 kilojoules (kJ).

What is a Kilocalorie?

A kilocalorie equals 1000 calories, and its abbreviation is "kcal".

The process that is used to calculate calories is called calorimetry. The process measures how much heat is released when a food sample is burned in a bomb calorimeter. This device is a sealed container filled with oxygen, surrounded by water.

The food is burned inside the device, and the heat released through combustion is absorbed into the surrounding water. Scientists measure the change in water temperature and calculate the amount of energy the sample releases.

How Are Calories Measured?

There is a simplified way to calculate calories. Most nutritionists and food manufacturers estimate the calories in foods and drinks by their carbohydrate, fat and protein content. Each macronutrient has its own value.

By multiplying the amount of each macronutrient in food by its caloric value, the total number of calories in food can be estimated. This method of calorie calculation is called the Atwater system, and it has been used for over a century to estimate the calorie content of foods.

Each macronutrient has a specific caloric value:

1 gram of protein contains 4 kilocalories

1 gram of carbohydrates contains 4 kilocalories

1 gram of fat contains 9 kilocalories

1 gram of alcohol contains 7 kilocalories

What Are Macronutrients and Micronutrients?

Macronutrients are what our diet is mainly made up of. Macronutrients are often referred to as "macro" or "macros". Macronutrients are the three main categories of nutrients our body needs and are considered essential. The categories are carbohydrates, fats and protein.

Carbohydrates include glucose, the primary energy source for our brains and muscles. Some healthy sources include fruit, vegetables, whole grains and legumes (e.g. lentils, chickpeas, black beans, kidney beans).

Proteins are used for building and repairing our tissue and assisting in producing hormones. Protein needs depend on many factors, such as age, gender, body structure, pregnancy, physical activity and health conditions.

The more physically active you are, the more protein your body needs. A sedentary adult's average recommended daily protein intake is 0.8 grams per kilogram of body weight. Heavily active people need between 1.2 g – 1.7 g. Some healthy protein sources are legumes, tempeh, tofu, edamame, broccoli and nutritional yeast.

In contrast, micronutrients are nutrients required by our bodies in smaller amounts. They include vitamins, minerals, and trace elements like iron, zinc, and selenium. Micronutrients are involved in metabolic processes such as energy production, immune system function, and growth.

They are essential for healthy bones, skin, hair, brain function, vision, and overall wellness. To find out the best foods to eat daily according to Dr Michael Greger read my other blog post on What Should You Eat Daily?

Examples of plant protein per 100g

* Amounts may vary – be sure to read the information on the packaging

  1. Red split lentils dried 25 g

  2. Green lentils dried 24 g

  3. Lentils canned 7 g

  4. Kidney beans dried 24 g

  5. Kidney beans canned 7 g

  6. Firm tofu 20 g

  7. Chickpeas dried (garbanzo beans) 19 g

  8. Chickpeas canned 6 g

  9. Tempeh 19 g

  10. Edamame 11 g

There has been a lot of controversy around whether fat consumption in our diet is healthy. Fats are high in calories and should be consumed in moderation and minimised if you are trying to lose weight. Unprocessed foods that contain fats provide the body with nutrients and fat that they contain.

Olives, for example, contain around 145 calories in 100 grams of olive. Olive oil is a highly processed food. The oil extracted from olives contains almost all the calories in olives but far fewer other nutrients. For the same 100-gram mass, olive oil contains 885 calories!

On a positive note, fat can help us absorb some vitamins. Fat-soluble vitamins include vitamins A, D, E and K. If you want to consume fats try eating whole plant-based sources such as nuts, seeds and avocado.

What is a Kilojoule?

A kilojoule (kJ) is another unit measuring energy used in physics. It is based on the International System of Units (SI). It calculates the energy in food and drinks and expenditure during physical activity. One kilojoule equals 1,000 joules.

In some countries, including Australia and New Zealand, kilojoules are the standard unit of energy used on food labels (nutritional labels) to provide information about the energy content of packaged foods and drinks. In other countries, such as the United States, calories are the standard unit of energy used on food labels.

Kilocalorie vs Kilojoules

Kilocalories and kilojoules are used interchangeably, and in some countries like Australia, they are often displayed side by side on the packaging. 1 kilojoule is equal to 1000 joules. They are both units of measurement for calculating energy.

In the food industry, the term kilocalorie is frequently referred to as just "calorie"; however, this can lead to confusion, especially when "calories" are presented against kilojoules rather than "joule".

How to Convert Kilojoules to Kilocalories

1 kilocalorie (kcal) is equal to 4.184 kilojoules (kJ).

1000 calories are equal to 4184 joules.

Plantz Matter correct label kcal
Correct Labelling
snip from Bing calories

You will often see food labelling where 1 calorie = 4.184 kilojoules, but this needs to be corrected. The label should read 1 kcal = 4.184 kJ.

Here, on a chocolate wrapper, the energy content is incorrectly presented as 509 kJ per serve = 122 cal per serve instead of 122 kcal. The food industry regularly uses the term "calorie" interchangeably with "kilocalorie", so bear this in mind whenever you read a food label. In food, a calorie is a tiny amount of energy.

It makes more sense to talk in kilocalories, and that's what we do. People have become used to them being referred to as just "calories" in the food industry. Still, the numbers shown are almost always the number of "kilocalories", not the correct number of calories. Imagine if we started using "metre" instead of "millimetre" just for convenience. Total chaos!

Plantz Matter incorrect label
Incorrect Labelling
snip from Bing unit conversion
snip from Bing unit conversion

Exercise Sessions and Energy Expenditure

Knowing your energy expenditure, or how much energy you use in calories or kilojoules can help you improve your workouts. The average person burns a certain number of calories every minute. The number depends on their size, biology, and exercise intensity.

For those who want to build more muscle, it is important to track these energy numbers. They show how much protein and other foods to eat. This is part of managing weight and maintaining a healthy weight. Muscle burns more calories than fat. It does so even when you’re just sitting.

Weight Management and Caloric Intake

Managing weight means watching the calories in food. We must also understand how they turn into kilojoules. People who want to lose weight must balance their energy intake with their daily energy needs. They must balance it with the calories they burn.

People burn calories through daily activities and exercise.

The amounts of energy needed daily varies from person to person and will change throughout their lives. The size of a person and their muscle mass can affect how many calories or kilojoules they burn. Knowing how to turn calories into kilojoules can clarify how much energy we use or eat. This can be helpful when following health guides, like the Nutrition Journal.


Calorie vs. Kilojoule in Everyday Life

It's good to know the difference between a calorie and a kilojoule. They both tell us how much energy our food gives. A calorie is what we use to measure this energy. If you want to heat a tiny amount of water by one degree, it takes one calorie of energy. A kilojoule is another way to measure energy, and it’s a bit bigger than a calorie.

One kilojoule is the same as about 0.24 calories. When we change calories to kilojoules, we multiply the number of calories by 4.184. This is helpful to know, especially when we talk about food and exercise energy. People like Edward Atwater worked a lot on understanding how we measure this energy. They also studied how water’s temperature can change with different amounts of heat.


If you would like to learn more about calories read What is Low Calorie Density. If you are more interested in figuring out what your calorie intake should be read How Many Calories Should I Eat a Day? | Harris-Benedict Equation. Thank you for reading. If you found this post interesting, please share.

* Calculator snippets taken from Bing


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