Labels often show energy in kilojoules and kilocalories, and large chains in England must print calories on their menus. Behind those numbers sits a nineteenth-century heat unit and a short table of factors set in law.
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1 kcal ≈ 4.2 kJ
kcal and kJ: same energy
Fat 9, protein 4
2000 kcal is a reference
Read the label with confidence
THE SHORT ANSWER
Same energy, two units.
A food calorie is a kilocalorie. Kilojoules measure the same energy in SI units.
THE UNIT
Heat, measured
A small calorie is roughly the energy to warm 1 gram of water by 1 °C. The food calorie is a thousand of them: one kilocalorie, which NIST lists as 4.184 kilojoules.
General information only, not medical advice. For advice about your own health, speak to a doctor, pharmacist or other qualified health professional.
A QUICK NUTRIENT CHECK
How much energy in a gram?
Pick a nutrient to see the conversion factor UK food labels use to work out energy. The bar shows how energy-dense each gram is.
Factors from Annex XIV of retained Regulation (EU) 1169/2011 on food information to consumers. This is general information, not dietary or medical advice.
TWO NUMBERS WORTH KNOWING
One unit, one reference point.
One figure that links the two units, one that menus in England must print.
4.184 kJ
Kilojoules in one kilocalorie
NIST's Guide to the SI lists the thermochemical kilocalorie as exactly 4,184 joules. The International Table kilocalorie is slightly larger, at 4,186.8 joules.
CONVERSION · NIST
2000 kcal
The statement large chains must show
Since 6 April 2022, businesses in England with 250 or more employees must show calories and the statement ‘adults need around 2000 kcal a day’ where customers choose food.
MENUS · ENGLAND
As a guide, the NHS says an average man needs 2,500 kcal a day and an average woman 2,000 kcal, and that needs differ with age, weight, height and how much exercise you do.
FOUR THINGS TO LOOK FOR
Read the energy line with confidence.
Labels and menus follow set rules, so a few checks tell you what a number actually refers to.
Find the “energy” row
On packaging, calories appear under the energy heading, often as kJ and kcal side by side.
Check what the number covers
Values may be per 100 g or per portion. On menus, calories refer to the portion described, such as one slice.
Convert if you need to
Divide kilojoules by about 4.2 to get kilocalories, or multiply kilocalories by 4.2 for kilojoules.
Treat 2000 as a reference
The menu statement is a general reference for adults. The NHS says individual needs vary.
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A LITTLE HISTORY
From steam lectures to the menu board.
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The word comes from the Latin calor, heat. Nicolas Clément introduced the calorie as a unit of heat in lectures between 1819 and 1824, and in 1887 Wilbur Atwater used the large Calorie in an influential article on the energy in food.
The joule became the SI unit of energy in 1948, and the calorie is not an SI unit. Food labels still use both, which is why the same energy often appears twice on a pack.
Questions
Questions people ask
Is a food calorie the same as a kilocalorie?
Yes. The calorie used for food is the large calorie, a kilocalorie, equal to 1,000 small calories. Labels in the UK show it as kcal.
What do kJ and kcal mean on food labels?
Both measure energy. kcal is the food calorie, a kilocalorie; kJ is kilojoules, the SI unit. One kcal is about 4.2 kJ (NIST lists 4.184). The NHS notes calorie information is often given in kcal and may also be given in kJ.
Where does '9 calories per gram of fat' come from?
Food labelling law sets fixed conversion factors for calculating energy: 37 kJ or 9 kcal per gram of fat, 17 kJ or 4 kcal for protein and most carbohydrate, 29 kJ or 7 kcal for alcohol and 8 kJ or 2 kcal for fibre.
Who has to put calories on menus?
In England, businesses with 250 or more employees selling non-prepacked food for immediate consumption, such as restaurants, cafes and takeaways, must show calories at the point of choice. The rules started on 6 April 2022.
Does everyone need 2000 kcal a day?
No. The menu statement is a general reference for adults. The NHS gives 2,500 kcal for an average man and 2,000 kcal for an average woman as a guide, and says needs depend on age, weight, height and activity.
