Calories versus kilojoules on food labels
The calorie on a food label is really a kilocalorie (kcal), and 1 kcal equals 4.184 kilojoules (kJ), the standard unit of food energy.
Remember
On a food label, calorie means kilocalorie. To get kilojoules, multiply the kcal figure by about 4.2, and put both energies into the same unit before you compare them.
Memory aid
giving one distance in miles and kilometres
Kcal and kJ describe the same energy, just as miles and kilometres describe one distance.
The calorie printed on food packaging is really a kilocalorie (kcal), shortened to calorie in everyday speech. A kilocalorie is roughly the energy it takes to warm one kilogram (a thousand grams) of water by 1°C. The calorie of physics is much smaller, because it warms just 1 g of water by 1°C, so one kilocalorie is 1,000 of those small calories. The "kilo" in kilocalorie is the clue: it means one thousand.
The kilojoule (kJ) is the standard unit of food energy, and it equals one thousand joules, where the joule is the standard scientific unit of energy. Labels that show both kcal and kJ are describing the same energy. The conversion is 1 kcal = 4.184 kJ, often rounded to 4.2 kJ.
To convert kcal to kJ, multiply by 4.184, or by about 4.2 for a quick estimate. For example, 100 kcal is about 420 kJ. Check the direction: the kJ result should be a little over four times the kcal figure, so a kJ result smaller than the kcal figure means the conversion was done the wrong way round. To go from kJ back to kcal, divide by about 4.2.
Before comparing labels, convert both energies to the same unit. Comparing 250 kcal with 1,250 kJ by the raw numbers makes the kJ food look like five times the energy of the kcal food, but 250 kcal is about 1,050 kJ, which is about 200 kJ less than 1,250 kJ.
Key facts
- Food label calorie
- a kilocalorie (kcal), roughly the energy needed to warm 1 kg of water by 1°C
- Small calorie
- warms 1 g of water by 1°C, so 1 kcal is 1,000 of them
- Conversion
- 1 kcal = 4.184 kJ, often rounded to 4.2 kJ
- Direction check
- a kJ figure comes out a little over four times its kcal figure
- Standard unit
- the kilojoule (kJ) is the standard unit of food energy
- Worked example
- 250 kcal is about 1,050 kJ, so it is less energy than 1,250 kJ
Exam traps
- Dividing a kcal figure by 4.184 to reach kilojoules goes the wrong way, because the kilojoule is the smaller unit, so the kJ figure has to be larger than the kcal figure.
- Defining the calorie by one gram of water gives the small calorie of physics, which is 1,000 times smaller than the calorie on a food label (a kilocalorie), because the food version is based on one kilogram of water.
- Treating the calories on a food label as kilojoules understates the energy involved by a factor of about four, since a kilocalorie is worth about 4.2 kJ.
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