Calories to Megajoules (cal to MJ)

1 calorie equals 0.000004184 megajoules exactly.

1 calorie = 0.000004184 megajoules

How to convert calories to megajoules

To convert calories to megajoules, multiply the number of calories by 0.000004184. This factor is exact. To go the other way, multiply megajoules by 239,005.7361 (rounded).

megajoules = calories × 0.000004184

Example: 10 calories = 0.00004184 megajoules.

The 'calorie' on food labels is really a kilocalorie. The thermochemical calorie (4.184 J) is used here unless marked IT.

About these units

Calorie (cal)

ScientificTraditional unit (still used in some regions)

The thermochemical calorie: exactly 4.184 joules, the energy to raise one gram of water by one degree Celsius. Not the same as the food Calorie, which is a kilocalorie.

Latin calor, meaning heat, supplied the name. Nicolas Clement used it in lectures on heat engines between 1819 and 1824, and by 1852 Pierre Favre and Jean Silbermann applied it to the small unit based on a single gram of water warmed by one degree Celsius. Because the specific heat of water varies slightly with temperature, early calories differed from one laboratory to the next. A mean value near 4.1855 J was published for the 15 degree calorie in 1950, with an uncertainty of 0.0005 J.

Chemists settled the matter by definition. The thermochemical calorie is exactly 4.184 J, which is the figure used here, and NIST lists it with that exact value. The calorie is not an SI unit and is regarded as obsolete, since the ninth General Conference on Weights and Measures in 1948 preferred the joule for heat. One thousand of these calories make a kilocalorie, the larger unit used for food, and a slightly larger calorie, the International Table version of 4.1868 J, is used in steam tables.

1 calorie = 4.184 joules (exactly).

Sources: Wikipedia: Calorie; NIST SP 811, Appendix B.8 (energy conversion factors)

Megajoule (MJ)

Metric (SI)Current unit

One million joules. Used for fuel energy content and natural gas billing in many countries.

Picture a one-tonne car moving at 44.7 m/s, about 100 miles per hour: its kinetic energy is roughly one megajoule. Because the joule was originally conceived in 1882 as 10 million ergs, a megajoule is 10 to the 13th ergs, a figure that shows how little the old CGS unit suited everyday work and why the joule won out.

Food provides a more familiar benchmark. A Snickers bar of 280 food calories holds about 1.2 MJ, and a day's 2,000 food calories for a moderately active woman come to about 8.4 MJ. A kilowatt-hour of electricity is exactly 3.6 MJ, and exploding a kilogram of TNT releases about 4.2 MJ.

In other words the megajoule sits at the scale of a single serving of fuel, food or electricity, which is why it turns up in energy-content tables where joules would give unwieldy numbers.

1 megajoule = 1,000,000 joules (exactly).

Sources: Wikipedia: Joule; Wikipedia: Orders of magnitude (energy); Wikipedia: Kilowatt-hour

Related conversions

See all energy units.