Calories and kilojoules are both units of energy. Specifically, they measure the energy content in food. When you see calorie counts on nutrition labels, those are actually kilocalories or Calorie (with a capital C). A kilocalorie is the same as a Calorie (with a capital C). This is different from the lower-case calorie that physicists use to measure the energy needed to raise one gram of water by one degree Celsius. So for nutrition purposes, calories actually refer to kilocalories.
Kilojoules are another metric unit used to measure energy, primarily outside the United States. To convert between calories and kilojoules, you need to know that 1 calorie is equal to 4.184 kilojoules. So to convert calories to kilojoules, you simply multiply the calorie amount by 4.184. To go the other way, from kilojoules to calories, you divide the kilojoule amount by 4.184.
More Details on Calories
When we talk about calories on a nutrition label, we’re referring to kilocalories or Calories (capital C). One Calorie is the amount of energy needed to raise the temperature of 1 kilogram of water by 1 degree Celsius. This is equivalent to 4.184 kilojoules.
The reason we use Calories rather than the smaller calories is because the amounts of energy in food are so large. To make the numbers easier to work with, we go up to the kilocalorie level. For example, a gram of protein contains 4 Calories. Using the smaller calories, that would be 4,000 calories per gram. Dealing with numbers in the thousands makes calculations more difficult. So nutritionists adopted the convention of using kilocalories and referring to them as Calories (capital) or sometimes kcals.
The calorie content listed on food and beverage labels refers to the Calories (capital C) or kilocalories. So when you see a label that lists 100 calories, it’s referring to 100 kilocalories or 100 Calories.
Kilojoules as a Measure of Food Energy
While Calories are the standard unit used for food labels in the United States, many other countries use kilojoules instead. The kilojoule (symbol kJ) is the metric unit for energy. Specifically, one kilojoule is the amount of energy needed to raise the temperature of 1 liter of water by 1 degree Celsius.
Kilojoules are commonly used to measure energy content of foods in Australia, Europe, and many other parts of the world. So if you come across a food label with kilojoules listed, that is another way of quantifying the energy content.
The main thing to understand is that kilocalories (Calories) and kilojoules are both metrics for energy. They can be converted between one another using a simple mathematical relationship.
The Relationship Between Calories and Kilojoules
Now that we understand calories and kilojoules both measure energy, how exactly do they relate? Here is the conversion:
1 Calorie (kilocalorie) = 4.184 kilojoules
So to convert between them:
– Calories to kilojoules: Multiply calories by 4.184
– Kilojoules to calories: Divide kilojoules by 4.184
Some example conversions:
– 200 Calories x 4.184 = 836.8 kj
– 500 kj / 4.184 = 119.5 Calories
This relationship allows easy conversion between food labels using either unit.
Another way to remember the conversion is that 1 Calorie raises 1 kilogram of water 1 degree Celsius, while 1 kilojoule raises 1 liter of water 1 degree Celsius. Since 1 kilogram is equal to 1 liter, they describe the same amount of energy.
Converting Between Calories and Kilojoules in Foods
When looking at food labels, you may come across calories or kilojoules. To make conversions:
– For calories to kilojoules: Multiply the calories by 4.184
– For kilojoules to calories: Divide the kilojoules by 4.184
Let’s look at some examples for different foods:
– A granola bar with 150 calories x 4.184 = 627.6 kj
– A serving of yogurt with 400 kj / 4.184 = 95.5 calories
– An apple with 95 calories x 4.184 = 397.5 kj
So for the granola bar, yogurt, and apple examples above, we were able to easily convert between calories and kilojoules.
This works for any food item. Just remember to multiply calories by 4.184 to get kilojoules, or divide kilojoules by 4.184 to get calories.
Converting Calories to Kilojoules in Daily Intake
We can also use the simple conversion factor to look at daily recommended energy intake values.
For example, the recommended daily calorie intake for an average moderately active woman is about 2,000 calories. To convert that to kilojoules:
2,000 calories x 4.184 kj/calorie = 8,368 kj
So the recommended average daily intake for a moderately active woman is 8,368 kj.
For an average moderately active man, the recommended intake is about 2,500 calories per day. Converting to kilojoules:
2,500 calories x 4.184 kj/calorie = 10,460 kj
Therefore, the average recommended daily intake for a moderately active man is 10,460 kj.
This demonstrates how easy it is to convert calories to kilojoules even for total daily energy intake recommendations using the 1 calorie = 4.184 kj relationship.
Real World Examples of Food Labels
Here are some examples of what real world food labels look like using either calories or kilojoules to measure energy content:
Calories (Nutrition label from United States)
| Serving Size | 1 cup (228g) |
|---|---|
| Calories | 250 |
Kilojoules (Nutrition label from Australia)
| Serving Size | 1 cup (250g) |
|---|---|
| Energy | 1,100kJ |
To convert between the two labels:
– 250 calories x 4.184 kj/calorie = 1,046 kj
– 1,100 kj / 4.184 kj/calorie = 263 calories
As you can see, real food labels from around the world display energy content in either calories or kilojoules. Using the simple math relationship of 1 calorie = 4.184 kj allows easy conversion back and forth.
Exceptions and Special Cases
When converting calories to kilojoules, it’s important to keep in mind a few special cases:
– The calories to kilojoules conversion only holds exactly when referring to kilocalories. For the smaller calories, 1 calorie = 4.184 joules (not kilojoules). But for nutrition purposes, we are always talking about kilocalories so the conversion to kilojoules is valid.
– Some regions may use Calories with a capital C instead of spelling out kilocalories. But the capital C still stands for kilocalories and the conversion to kilojoules still applies.
– In some older textbooks or publications, you may see the use of calorie with a lower case c to refer to kilocalories. However, the more recent convention is to distinguish them by always using capital C Calorie for kilocalories.
– The Calories on food labels actually measure metabolizable energy, not gross energy. This takes into account that we do not digest or absorb 100% of the energy. However, the difference is small enough that we still use the 4.184 kj/Calorie conversion.
So in most practical cases, you can accurately convert Calories (or kcals or capital C calories) to kilojoules by multiplying by 4.184. Just remember that this refers specifically to the kilocalories rather than smaller calories.
Performing Conversions in a Spreadsheet
When you need to convert a large list of calorie values to kilojoules or vice versa, the easiest way is to set up a spreadsheet to perform the calculations.
For example:
| Food | Calories | Kilojoules |
|---|---|---|
| Apple | 95 | =B2*4.184 |
| Yogurt | 150 | =B3*4.184 |
| Granola Bar | 200 | =B4*4.184 |
In this spreadsheet, column B contains the calories. Column C uses a formula that multiplies column B by 4.184 to automatically calculate the number of kilojoules.
You can quickly copy or fill this formula down Column C for hundreds or thousands of rows to efficiently perform mass conversions from calories to kilojoules.
Of course, you can also use the same approach to create a column that automatically converts kilojoules to calories by dividing by 4.184 instead. Spreadsheet tools make conversion quick and easy!
Finding Conversion Factors Online
In addition to memorizing that 1 calorie = 4.184 kilojoules, you can also find conversion factors between many units using online tools and resources:
– Google: Simply search for “calories to kilojoules” and you will immediately get the conversion factor
– UnitConverter.net: Provides conversions for hundreds of units
– WolframAlpha: A sophisticated computational knowledge engine that can handle many unit conversions
So if you ever need to double check or look up the conversion factor between calories and kilojoules, or any other units, there are many free online tools and resources available. No need to memorize all the factors, simply look them up when needed.
Don’t Convert Between Calories and Joules
One mistake people sometimes make is trying to convert between calories (meaning kilocalories) and joules. Joules are only equivalent to the smaller calories, so this conversion does not work.
Remember:
– Kilocalories (Calories) should only be converted to kilojoules
– Smaller calories convert to joules
Trying to directly relate Calories to joules will result in very large incorrect numbers. Always match the prefixes when converting energy units.
The takeaway is to never try converting Calories or kcals directly to joules. Use kilojoules for that conversion. And conversely, only convert the smaller calories to joules, not Calories or kilocalories. Keeping the prefixes matched avoids mistakes.
Being Careful with Terminology
In summary, always be careful about terminology when converting between energy units:
– The Calories on nutrition labels actually refer to kilocalories. This is different from the smaller calories used in physics.
– To convert nutrition Calories or Food Calories, only use kilojoules, not joules.
– Lowercase calories should only be converted to joules, not kilojoules
– Using lowercase c for calorie vs uppercase C for Calorie avoids confusion, but some older sources may not follow this convention.
– Understand that kcals, Calories, calorie, and kilocalories all refer to the same kilocalorie unit.
By being careful with the terminology and matching prefixes, you can accurately convert between nutritional Calories and kilojoules. Just remember that food Calories are really kilocalories and therefore convert to kilojoules, not joules.
Conclusion
In summary, calories and kilojoules are two units used to measure the energy content in foods. While Calories (capital C) are more often used on nutrition labels in the United States, kilojoules are commonly used in many other countries.
The conversion factor between nutrition Calories and kilojoules is:
1 Calorie = 4.184 kilojoules
To convert calories to kilojoules, multiply the calorie amount by 4.184. To go the other way, from kilojoules to calories, divide the kilojoule amount by 4.184.
It’s important to remember that food Calories actually refer to kilocalories, not the smaller calories used in physics. So make sure to only convert them using kilojoules, not regular joules.
With this straightforward relationship of 1 Calorie = 4.184 kj, you can easily convert energy amounts between food labels using either unit. Spreadsheets also provide a handy way to do mass conversions between the units. Paying attention to the terminology and proper prefixes allows accurate conversions between these common measures of food energy.
