Joules to Calories Converter

4.184 joules make one calorie — and the calorie on a food label is a thousand of them.

Conversion table

Energy	In calories
10	2.3901 cal
25	5.9751 cal
50	11.9503 cal
75	17.9254 cal
100	23.9006 cal
150	35.8509 cal
200	47.8011 cal
250	59.7514 cal
300	71.7017 cal
400	95.6023 cal
500	119.5029 cal
600	143.4034 cal
750	179.2543 cal
1,000	239.0057 cal
1,500	358.5086 cal

🥗 Numbers on a label

1 cal4.184 J
1 kcal (1 food Calorie)4,184 J = 4.184 kJ
100 kcal418.4 kJ
2,000 kcal8,368 kJ

Reading energy on food

The Calorie on a nutrition label is a kilocalorie. A 250 Calorie snack contains 250,000 physics calories, or about 1,046 kilojoules.
European labels are required to print kilojoules and kilocalories together, so the two figures on a packet are never contradictory — just the same energy twice.
To go from kJ to kcal in your head, divide by four and take off a little; 2,000 kJ is about 478 kcal.
Australia and New Zealand quote kilojoules first, which is why an Australian menu showing 2,000 next to a burger is not an error.

What a calorie is, and why 4.184

A calorie was originally defined as the heat needed to raise one gram of water by one degree Celsius. That turned out to be awkward, because water takes slightly different amounts of heat at different starting temperatures, so several rival calories existed — the 15 °C calorie, the mean calorie, the international steam-table calorie. The thermochemical calorie settled the argument by definition: it is exactly 4.184 joules, no measurement required. The joule, by contrast, comes from mechanics — one newton acting through one metre — and the fact that the two connect at all is what Joule's paddle-wheel experiments established in the 1840s.

The food Calorie is a thousand calories

Nutrition adopted the kilocalorie because the physics calorie is far too small to be useful: an apple would be 95,000 of them. In American usage the kilocalorie is written with a capital C — one Calorie equals one kilocalorie equals 4,184 joules — but that typographic distinction disappears the moment anyone writes it by hand or says it out loud. Europe sidestepped the problem entirely: EU labelling rules require both kilojoules and kilocalories on every packet, so a bar showing 2,301 kJ and 550 kcal is stating the same energy twice, once in SI and once in the unit people are used to.

Reading the numbers on a packet

EU labels show energy per 100 g in both kJ and kcal, which makes products directly comparable regardless of pack size.
The United States prints Calories only, and always means kilocalories.
Australia and New Zealand lead with kilojoules; the daily reference intake there is quoted as 8,700 kJ.
The kJ and kcal figures on the same label always agree: divide the kJ by 4.184 to check.
Label energy is calculated from macronutrients using standard factors, not measured in a bomb calorimeter for each batch.
Fibre and sugar alcohols carry reduced energy factors, which is why some products show fewer calories than a simple sum would suggest.

What these numbers mean for a body

A typical adult reference intake is 2,000 kcal a day, which is 8,368 kJ or 8.37 megajoules.
One gram of fat supplies 9 kcal (37 kJ); protein and carbohydrate supply 4 kcal (17 kJ) each.
Running one kilometre costs roughly 60 kcal for a 70 kg adult, about 250 kJ.
A 100 g bar of milk chocolate is around 550 kcal, or 2,301 kJ — more than a quarter of a day.
Basal metabolism, the energy used just staying alive, is typically 1,300–1,800 kcal a day.
A kilowatt-hour of electricity is 860 kcal, roughly the energy in a large restaurant main course.

Worked examples

Between the laboratory and the label

A snack showing 1,046 kJ

1,046 kJ ÷ 4.184 = 250 kcal
That is 250 food Calories
In physics calories: 250,000 cal

1,046 kJ = 250 kcal

A 2,000 kcal daily intake

2,000 kcal × 4.184 = 8,368 kJ
= 8.368 MJ
Or 2.32 kWh, about the energy of a hot bath

2,000 kcal = 8,368 kJ

Frequently Asked Questions

How many joules are in a calorie?

4.184 joules exactly, for the thermochemical calorie used in nutrition and chemistry. The value is a definition rather than a measurement, which is why it carries no uncertainty. A kilocalorie — the Calorie on a food label — is 4,184 joules.

Is a food Calorie the same as a calorie?

No: a food Calorie is a kilocalorie, a thousand physics calories. American labels write it with a capital C to mark the difference, but that convention vanishes in handwriting and speech. A 500 Calorie meal contains 500,000 calories in the strict sense, or 2,092 kilojoules.

How do I convert kJ to kcal?

Divide by 4.184. So 2,000 kJ is 478 kcal and 1,000 kJ is 239 kcal. For a quick mental estimate, divide by four and subtract a few per cent; going the other way, multiply kcal by four and add a little.

Why do European labels show both kJ and kcal?

Because EU food information rules require it. Kilojoules are the SI unit, kilocalories are what shoppers recognise, and printing both avoids a choice between correctness and familiarity. The two figures on a packet always describe the same energy.

How many kJ should I eat per day?

The reference intake used on European labels is 8,400 kJ, or 2,000 kcal, for an average adult; Australia and New Zealand use 8,700 kJ. These are labelling references rather than personal targets — real needs vary with body size, age and activity, often by several hundred kilocalories either way.

Why is the calorie not an SI unit?

Because the SI already has the joule for energy, and the calorie was tied to a property of water that is not constant. Several incompatible calories existed, which is exactly the problem SI units are meant to remove. It survives in nutrition through familiarity, which is why labels increasingly carry the joule figure alongside it.