Joules to Calories Calculator

About the Joules to Calories Calculator

The joule is the scientific unit of energy, used in physics, chemistry and on food labels in many countries as kilojoules. The calorie is an older unit that survives in nutrition, where a food "Calorie" is really a kilocalorie, and in some areas of chemistry and engineering. Students, dieters, athletes and anyone reading a label printed in the other unit often need to convert between the two.

This joules to calories calculator converts joules to calories exactly. It also gives kilocalories (food Calories) and kilojoules, the slightly different International Table calorie used in engineering, and watt-hours for comparison with electrical energy.

How to Use the Joules to Calories Calculator

Enter the energy in joules. For kilojoules, multiply by 1,000 first: 2,000 kJ is 2,000,000 J.

The calories appear straight away, with kilocalories, kilojoules, International Table calories and watt-hours.

The Formula

  calories      = joules ÷ 4.184          (thermochemical calorie)
  kilocalories  = joules ÷ 4,184          (food Calorie)
  IT calories   = joules ÷ 4.1868         (International Table calorie)
  watt-hours    = joules ÷ 3,600

The thermochemical calorie of exactly 4.184 joules is the standard in chemistry and nutrition labelling. The International Table calorie is 4.1868 joules, about 0.07 percent larger, and is used in some steam and engineering tables.

Step-by-Step Example

1,000 joules.

  1,000 ÷ 4.184 = 239.006 cal
  239.006 ÷ 1,000 = 0.239006 kcal

So 1 kilojoule is 0.239006 kilocalories. This is the factor behind every food label that shows both units.

One food Calorie is 4,184 joules, or 4.184 kJ; a 2,000 kcal daily diet is 8,368 kJ.

Quick Reference

  1 J        = 0.239006 cal
  4.184 J    = 1 cal
  1 kJ       = 0.239006 kcal
  4.184 kJ   = 1 kcal (1 food Calorie)
  1,000 kJ   = 239.006 kcal
  1 Wh       = 860.421 cal = 0.860421 kcal
  1 kWh      = 860.421 kcal

Calories and Calories

The word calorie is used in two ways. The small calorie, written cal, is the energy needed to warm one gram of water by about one degree Celsius — 4.184 joules. The large Calorie, written with a capital C, kcal or Cal, is 1,000 small calories, the amount needed to warm a kilogram of water by one degree. Food labels, fitness trackers and diet plans always mean the large Calorie, even when they write it with a small c. A 250-calorie snack contains 250 kilocalories, about 1,046 kilojoules.

Food Labels Around the World

In the EU, UK, Australia and New Zealand, nutrition labels show energy in kilojoules, often with kilocalories alongside. In the US and Canada, labels show only Calories. To compare products from different countries, convert to the same unit: 100 grams of a cereal labelled 1,600 kJ contains about 382 kcal. Australians in particular often count kilojoules rather than calories when dieting.

Energy in Everyday Terms

Joules are small. Lifting a 100 gram apple one metre takes about one joule. A 60 watt bulb uses 60 joules every second, 216,000 joules in an hour, which is about 51.6 kilocalories. Walking a kilometre uses roughly 200 to 300 kilojoules for an average adult, about 50 to 70 kilocalories. Seen this way, food energy is enormous: one chocolate bar of 250 kcal holds over a million joules.

Exercise and the Body

Fitness equipment and apps sometimes report work done in kilojoules and energy burned in kilocalories. Because the human body is only about 20 to 25 percent efficient, the food energy used during exercise is about four times the mechanical work done. That is why a cycling computer's figure for work in kilojoules is often roughly equal to the kilocalories burned: the factor of 4.184 and the body's efficiency of about a quarter nearly cancel out.

Heat in Chemistry and Physics

In the laboratory, heat is calculated with the formula q = m × c × ΔT, where m is the mass, c the specific heat capacity and ΔT the temperature change. Water's specific heat is 4.184 joules per gram per degree Celsius — exactly one calorie per gram per degree, which is how the calorie was defined. Warming 250 grams of water from 20 °C to 100 °C therefore takes 250 × 4.184 × 80 = 83,680 joules, or 20,000 calories, 20 kilocalories. Older chemistry textbooks often give energies of reaction in kilocalories per mole; multiply by 4.184 to convert them to the kilojoules per mole used in modern data tables.

Quick Mental Rule

A kilojoule is roughly a quarter of a kilocalorie. Divide kilojoules by 4 for a quick estimate that is about 4.6 percent high: 800 kJ is roughly 200 kcal, against the exact 191.2 kcal.

Understanding Your Result

The headline is the energy in small calories.

The kilocalories and kJ line gives food Calories and kilojoules.

The International Table calories line gives the engineering calorie, 4.1868 J.

The watt-hours line compares the energy with electricity use.

When Should You Use This Calculator?

Use it to compare food labels in kilojoules and kilocalories.

Use it for chemistry and physics problems involving heat.

Use it with fitness devices that report kilojoules of work.

Use it to compare food energy with electrical energy in watt-hours.

Common Mistakes

Confusing calories with Calories. Food Calories are kilocalories.

Mixing the two calories. The thermochemical calorie is 4.184 J; the IT calorie 4.1868 J.

Forgetting kilojoules. Multiply kJ by 1,000 before converting from joules.

Dividing by 4.2. It is close, but about 0.4 percent out.

Equating work with food energy. The body uses about four times the mechanical work done.

Reading kcal per mole as kJ per mole. Older tables in kilocalories need multiplying by 4.184 before they can be compared with modern kilojoule values.

Trusting rounded label figures too far. Food labels round energy values, so small differences between products may not be real.