Millijoule (mJ)

Metric (SI)Current unit

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

1 millijoule = 0.001 joules (exactly).

About the millijoule

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.

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

Millijoule conversion chart

1 millijoule expressed in every other energy unit:

More Metric (SI) units

• joule (energy)
• kilojoule (energy)
• megajoule (energy)
• gigajoule (energy)
• watt-hour (energy)
• kilowatt-hour (energy)
• megawatt-hour (energy)
• tonne of oil equivalent (energy)
• meter (length)
• kilometer (length)
• centimeter (length)
• millimeter (length)
• micrometer (length)
• nanometer (length)
• picometer (length)
• decimeter (length)

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