Many people think of the metric system and the International System of Units (SI) as being the same thing. In reality, the SI is the modern form of the metric system and represents the result of more than two centuries of refinement and standardization. During that time, numerous metric units were introduced, modified, replaced, or removed from official use.
Some of these units disappeared because improved scientific understanding led to better alternatives. Others were replaced as the scientific and engineering communities worked to establish a single coherent system of measurement. As a result, older technical literature often contains metric units that are unfamiliar to modern students and technicians.
The table below lists several metric units that were once common but are no longer preferred within the SI.
| Unit | Symbol | Value | Modern SI Alternative |
|---|---|---|---|
| Micron | μ | 10⁻⁶ m | Micrometer (μm) |
| Ångström | Å | 10⁻¹⁰ m | Nanometer (nm) or picometer (pm) |
| Dyne | dyn | CGS unit of force | Newton (N) |
| Erg | erg | CGS unit of energy | Joule (J) |
| Gauss | G | CGS magnetic flux density | Tesla (T) |
| Oersted | Oe | CGS magnetic field strength | Ampere per meter (A/m) |
| Maxwell | Mx | CGS magnetic flux | Weber (Wb) |
| Poise | P | CGS dynamic viscosity | Pascal-second (Pa·s) |
| Stokes | St | CGS kinematic viscosity | Square meter per second (m²/s) |
| Barye | Ba | CGS pressure | Pascal (Pa) |
| Kilogram-force | kgf | Gravitational force on a 1 kg mass | Newton (N) |
| Calorie | cal | Unit of energy | Joule (J) |
The micron provides a good example of how SI standardization changed terminology. Originally, the micron was the official name for one-millionth of a meter. When the SI adopted a systematic set of prefixes, the term micrometer became preferred because it follows the same naming convention as millimeter, kilometer, and nanometer. Although “micron” still appears in manufacturing, filtration, and vacuum technology, the SI unit is the micrometer (μm).
The ångström remains common in fields such as crystallography, spectroscopy, and atomic physics. One ångström is equal to 0.1 nanometer, making it a convenient unit for expressing atomic dimensions. While not part of the SI, it survives because many scientific measurements naturally fall near one ångström in size.
Many of the units in the table originated in the centimeter-gram-second (CGS) system, which was widely used during the nineteenth and early twentieth centuries. Under CGS, force was measured in dynes, energy in ergs, and magnetic flux density in gauss. As engineering and science became increasingly international, the meter-kilogram-second (MKS) system gained favor. This eventually led to the creation of the International System of Units (SI), which adopted the newton, joule, tesla, and other units that remain in use today.
Another category of abandoned metric units includes gravitational units such as the kilogram-force. Before SI, engineers often expressed loads, pressures, and stresses using kilogram-force rather than the unit of force now known as the newton. As a result, older machine drawings, textbooks, and engineering specifications may be entirely metric while still not using SI units.
Not every older metric unit has disappeared. The liter remains accepted for use with SI and continues to be widely used for liquids. Likewise, the hectare remains common in land measurement, and the metric tonne (1,000 kilograms) remains important in commerce and industry. These units are not SI units, but they are officially accepted for use alongside SI units.
The history of these units serves as a reminder that the metric system has evolved over time. The modern SI did not replace non-metric measurement systems alone; it also unified and standardized many competing metric systems that had developed during the previous two centuries.
Not all metric units that originated before the International System of Units (SI) have been replaced. While many historical units such as the dyne, erg, and kilogram-force have largely fallen out of use, several non-SI units remain officially accepted for use alongside the SI. These units are recognized by the International Bureau of Weights and Measures (BIPM) and are included in the SI Brochure because of their widespread use and practical importance.
Examples include the liter (L) for volume, the hectare (ha) for land area, the tonne (t) for mass, the electron volt (eV) for atomic and particle physics, the dalton (Da) for atomic and molecular masses, and the astronomical unit (au) for distances within the solar system. Time units such as the minute, hour, and day are also accepted for use with the SI despite not being SI units themselves.
These units occupy a unique position within the modern measurement system. They are not part of the SI, yet they remain internationally recognized and are routinely used in science, engineering, commerce, and everyday life. Their continued acceptance reflects a practical balance between maintaining a coherent system of measurement and preserving units that are deeply established within their respective fields.
SI Units
These are units that are actually part of the SI itself.
Examples include:
- meter (m)
- kilogram (kg)
- second (s)
- ampere (A)
- kelvin (K)
- mole (mol)
- candela (cd)
and derived units such as:
- newton (N)
- joule (J)
- pascal (Pa)
Non-SI Units Accepted for Use with the SI
These units are not SI units, but the BIPM permits and recommends their continued use alongside SI units because they are deeply entrenched and useful.
Examples currently listed in the SI Brochure include:
- minute (min)
- hour (h)
- day (d)
- liter (L)
- tonne (t)
- hectare (ha)
- electron volt (eV)
- dalton (Da)
- astronomical unit (au)
- degree (°), minute (′), second (″) for plane angle
The BIPM maintains official tables of these units in the SI Brochure.
Units Not Accepted for Use with the SI
Then there are units that are neither SI units nor accepted for use with SI.
Examples include:
- kilogram-force (kgf)
- dyne
- erg
- gauss
- oersted
- stokes
- micron (as a unit name)
These survive mainly for historical reasons or in specialized fields.