Calories to Millijoules (cal to mJ)

1 calorie equals 4,184 millijoules exactly.

1 calorie = 4,184 millijoules

How to convert calories to millijoules

To convert calories to millijoules, multiply the number of calories by 4,184. This factor is exact. To go the other way, multiply millijoules by 0.0002390057361 (rounded).

millijoules = calories × 4,184

Example: 10 calories = 41,840 millijoules.

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)

Millijoule (mJ)

Metric (SI)Current unit

One thousandth of a joule. Used for very small amounts of energy.

Prefixing the joule with milli gives a unit one thousandth of the base size, so 1 mJ is 0.001 J. The joule itself honours the English physicist James Prescott Joule, and its name was proposed by Wilhelm Siemens in 1882 before adoption at the 1889 International Electrical Congress. Because the joule equals one watt-second, a millijoule is also a milliwatt applied for one second, which makes it a natural size for tiny electrical budgets.

Some verifiable comparisons show the scale. One list of energy magnitudes puts an atomic battery's output over an hour at about 3 mJ, and an LED running for a second at about 40 mJ. A half-dollar coin dropped from a metre releases roughly 110 mJ. In atomic units, one millijoule is about 6.24 x 10^15 electronvolts, using the exact electronvolt value of 1.602176634 x 10^-19 J. Below the millijoule come the microjoule and nanojoule, so it sits at the coarse end of small-energy measurement.

1 millijoule = 0.001 joules (exactly).

Sources: Wikipedia: Joule; Wikipedia: Orders of magnitude (energy); Wikipedia: Electronvolt

Related conversions

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