Symbols for units begin with a capital letter when they are named after an individual (for example, ampere, A; kelvin, K; hertz, Hz; coulomb, C). Otherwise they always begin with a lower case letter (for example, metre, m; second, s; mole, mol).
SI Metric Base Quantities and Units
There are seven base quantities in the SI system, with the following units:
Derived Units with Special Names in the SI
Derived units are formed by combining the base units according to the algebraic relations linking the corresponding quantities. There is no set number of derived units, as the possibilities are endless. The below 22 derived units have special names for convenience of reference, as they are frequently used.
Non-SI Units Accepted for Use with the International System of Units
There are many non-SI units that are accepted for use with the International System, because they are widely used in everyday life, are of historical importance or provide an advantage or convenience in a particular scientific, technological or commercial specialty. Below are some non-SI units that are accepted for use with the International System because they are widely used with the SI in matters of everyday life. Their use is expected to continue indefinitely, and each has an exact definition in terms of an SI unit.
Numerous other Non-SI Units are accepted for use only in special circumstances. See the BIPM Brochure, International System of Units (SI).
The symbol for the liter is an exception to the rule of using capital letters for symbols of units named after an individual. Either a lower case letter or a capital L may be used, the capital being allowed in this case to avoid confusion between the lower case letter l (el) and the number one, 1 (one).
In English speaking countries, the tonne is usually called “metric ton”.