Liter (L)

The liter is the metric unit for drinks, fuel and most cooking liquids outside the US. Picture a box ten centimeters on each side: the space inside is one liter. In British English the word is spelled litre.

The liter has not always matched that box exactly. From 1901 it was defined as the volume of a kilogram of pure water, which made it about 28 parts per million larger than a cubic decimeter (NIST Handbook 44, Appendix B).

In 1964, the General Conference on Weights and Measures dropped the water definition and declared the liter a special name for the cubic decimeter (BIPM). It also advised against using the name for high-accuracy volume measurements.

Both symbols are correct. The General Conference on Weights and Measures added the capital L in 1979 because a lowercase l is easy to confuse with the number one (BIPM). In the United States, NIST's style guide uses L, along with the American spelling liter (NIST).

The same choice carries over to smaller units, so recipes and packages from different countries show both ml and mL. This site writes L for liters and ml for milliliters.