Even though I’m a dietitian in Australia where we use the metric system and kilojoules are the unit that is used on nutrition panels, I’ve always preferred to talk about calories in stead of kilojoules.
From my experience, that is also the way my clients and followers on social media typically prefer to talk as well.
I believe a large portion of this is because a lot of people read about nutrition online and a lot of the information comes out of places like America where calories are used instead.
That being said, even though most people prefer to talk in terms of calories, this still creates an issue when looking at nutrition panels that do not include the calorie number, for those who do not know how to do the conversion.
What Are Calories and Kilojoules?
Both calories and kilojoules are units of energy.
Calorie is the imperial system version and kilojoule is the metric system version.
And to be technical, when people are referring to “calories” the vast majority of the time they are actually referring to kilocalories, which is actually 1000 calories.
When you see the abbreviation “Cal”, this is also referring to kilocalories. Note that the ‘c’ should technically also be capitalised in this case to denote one thousand.
Without getting too bogged down in semantics, a lower-case ‘c’ in cal would mean only 1 kilocalorie.
2,000 Cal = 2,000 kilocalories
2,000 cal = 2 kilocalories
Despite this it is typically assumed ‘cal’ and ‘Cal’ are the same.
If somebody is talking about how a certain meal is 300 calories, they actually mean it is 300 kilocalories. It’s just a lot easier to say calorie than kilocalorie.
In terms of definition, a calorie was originally defined as the amount of heat needed to raise the temperature of one litre of water from 0 degrees to 1 degree Celsius.
This definition was later changed to be compared to joules instead, which are a unit that is more commonly used by scientists to describe the amount of work needed to force one newton through one meter.
This conversion is that 1 calorie (not kilocalorie) equals 4.18 joules. And 1 joule equals 0.24 calories.
How to Convert Calories to Kilojoules
One calorie (kilocalories) is 4.18 kilojoules. Or to put it differently, 1kcal = 4.18kj.
This therefore means that to convert 2000kcal into kilojoules, you would multiply 2000kcal by 4.18. This would make it 8,360kj.
If you had 10,000kj and wanted to figure out how many calories it is, you would just divide by 4.18. This would make it 2,440kcal.
If you want to make it slightly simpler, you could change the conversion to 4.2.
Or to make it even simpler, you could just round it to 4. This makes it much easier when you do not have a calculator.
For example, if something had 2,000kcal, quick maths would let you know that it is ~8,000kj. It’s only slightly off, but much quicker than getting a calculator out.
Calories and kilojoules are literally made up of macronutrients.
These macronutrients are protein, carbs, fat and alcohol.
The below figures are rounded numbers, but are helpful for figuring out roughly how many calories are coming from each macronutrient in a food.
Protein = 4kcal/g
Carbs = 4kcal/g
Fat = 9kcal/g
Alcohol = 7kcal/g
Therefore if a food has 10g protein and 10g carbs, but no fat or alcohol, it would have roughly 80kcal. This would be because you would multiply the protein amount (10g) by 4 (since protein has 4kcal per gram) and the carb amount (10g) by 4 (since carbs have 4kcal per gram.
Carbs = 16.7kj/g (or just 17kj/g)
Protein = 16.7kj/g (or just 17kj/g)
Fat = 37.7kj/g (or just 38kj/g)
If you have the time to do the accurate calculation, the conversion is 4.18. One calorie is 4.18 kilojoules.
If you just want a rough estimate, which is typically fine if you just want to figure out the rough amount of calories/kilojoules, you can get close enough with a conversion of 4. One calorie is just over 4 kilojoules.