NonSI

Non-SI units are units of measurement that are not part of the International System of Units (SI) but remain widely used in science, industry, and everyday life. They are often permitted for use with SI units by international standards, national regulations, or customary practice, though SI is typically the preferred system for scientific reporting.

Common non-SI units span several domains. For volume, the liter (L) is frequently used, with 1 L equal to 0.001 cubic metres. For mass, the tonne (t) equals 1000 kilograms. Time commonly uses the minute (min) and the hour (h). Temperature is often expressed in degrees Celsius (°C). In energy, the electronvolt (eV) is frequently used in physics. In chemistry and biology, the unified atomic mass unit (u or Da) is used for molecular masses. Lengths in certain technical fields use the angstrom (Å), while astronomy uses the astronomical unit (AU) and the parsec (pc). In meteorology and engineering, bar and atmosphere (atm) are traditional units for pressure, and the hectare (ha) is used for land area.

Usage guidelines emphasize that SI units provide the basis for measurements, with non-SI units allowed as supplementary or customary. When reporting values, many authors present SI values and may add the non-SI unit in parentheses or as a secondary label (for example, 5 L or 5 × 10^−3 m^3). Some non-SI units have exact conversion factors to SI units, established by standards bodies such as the BIPM, while others depend on field-specific conventions.

See also: International System of Units, metric system, unit conversion standards.