Common non-SI units

from Wikipedia, the free encyclopedia

The pursuit of standardized specification of physical quantities has led to the International System of Units (SI) . This is common all over the world, in most industrialized countries the use of SI units for official and business transactions is required by law.

For the European Union, Directive 80/181 / EEC of the European Council of December 20, 1979 stipulates the use of the International System of Units in business and official transactions. This was implemented into national law in Germany in the 1970s through the Units and Time Act , in Austria through the Measure and Verification Act , and in Switzerland there are corresponding regulations in the Federal Act on Metrology . In addition to the SI units, there are also other legal units . Since information in legal units is not required in many areas (e.g. in the field of research) or other units are permitted if they are given in addition to SI units, many such units are common in German-speaking countries. These common non-SI units are listed below.

Units approved for use with the SI

The International Bureau of Weights and Measures (BIPM) publishes in a brochure Le Système international d'unités  - also known as "the SI brochure" in German and adapted by PTB for Germany - in addition to the SI units, also the units that Are "approved for use with the SI".

The following complete list of units that are “approved for use with the SI” is arranged according to the associated size. Some of these units can also be used together with SI prefixes (such as “kilo” or “milli”).


  • Minute (min), hour (h) and day (d ; "standard day"):
    • 1 min = 60 s
    • 1 h = 60 min = 3600 s
    • 1 d = 24 h = 1440 min = 86 400 s

The use of SI prefixes with these three time units is not permitted according to the SI brochure. The year is not listed as an accepted unit, but is generally used, also in legal texts, e.g. B. when specifying times for dates and deadlines .


  • Astronomical Unit (abbreviated AE , international au for Astronomical Unit )
    • 1 AE = 149 597 870 700 m
    • The AE corresponds roughly to the mean distance between the earth and the sun .


  • Hectare (ha):
    • 1 ha = 100 a = 10 4  m 2 = 10 000 m 2
    • The (or the) hectare is a legal unit in Germany, Austria and Switzerland - there with the unit name “hectare” - but limited to the indication of the area of ​​land and parcels. According to the EEC guideline 80/181 / EEC, “hectare” is just a special name for one hundred times Ar , formed from the prefix hecto for 100 and the unit name Ar.


  • Liter (l or L):
    • 1 l = 1 L = 1 dm 3 = 1 000 cm 3 = 10 −3  m 3


  • Ton (t):
    • 1 t = 1 Mg = 10 3  kg = 1,000 kg
  • Atomic mass unit (u), also called dalton (Da)
    • 1 u = 1.660 538 782 (83)e-27 kg
    • The atomic mass unit is used to indicate atomic and molecular masses.


  • Electron volt (eV)
    • 1 eV = 1.602 176 634e-19 y
    • The electron volt (or "electron volt") is used to indicate energies in atomic, nuclear and particle physics.

Sizes of the dimension number

Other statutory non-SI units in the EU or Switzerland

Statutory non-SI entities in the European Union

Further units outside the SI are "legal units" according to Directive 80/181 / EEC . This is implemented essentially identically in national laws in all EU countries. For Germany, the Physikalisch-Technische Bundesanstalt (PTB) lists such non-SI units in the constantly updated brochure “The legal units in Germany”.


  • Ar (a):
    • 1 a = 100 m 2
    • The Ar is the legal unit in Germany and Austria for specifying the area of ​​land and parcels.
  • Barn (b):
    • 1 b = 100 fm 2 = 10 −24  cm 2 = 10 −28  m 2
    • The barn is used in nuclear physics as a unit for specifying effective cross sections.


  • Metric carat = 0.2 g
    • Mass of precious stones, especially diamonds
    • According to the EEC guideline, no unit symbol is specified for the metric carat.


  • Bar (bar)
    • 1 bar = 100 kPa = 10 5  Pa
  • Millimeter Mercury Column - (mmHg):
    • 1 mmHg = (101 325/760) Pa ≈ 133.322 Pa (only for blood pressure measurement ). The unit is the legal unit for specifying blood pressure and the pressure of other body fluids. In the EEC guideline, the unit symbol is written with a space: mm Hg, in the national laws of Germany, Austria and Switzerland as mmHg.


  • Kilowatt hour (kWh):
    • 1 kWh = 3.6 megajoules (MJ)
    • To determine the electricity and heating costs.


Refractive power of optical systems

Length-related mass

  • Tex (tex):
    • 1 tex = 1 g / km = 10 −6  kg m −1
    • Length-related mass of textile fibers and yarns.

Dimensionless sizes

  • Gon (grad)
    • 1 gon = (π / 200) rad
  • Full angle :
    • 1 full angle = 360 ° = 2π rad

Exceptions in Ireland

Other non-SI units are still valid for special purposes in those EU member states in which they were approved on April 21, 1973. Here are some of the Anglo-American units of measure in Ireland :

For volume information when serving beer and cider on tap and for milk in reusable containers:

  • Pint (pt):
    • 1 pt = 568.3 cm 3

For mass information when trading in precious metals:

  • Troy ounce , troy ounce (oz tr):
    • 1 oz tr = 31.1034768 g

Statutory non-SI units in Switzerland

The Swiss Unit Ordinance of November 23, 1994 largely follows the European regulation . However, there are the following differences:


  • Are (a) and hectares (ha):
    • 1 ha = 100 a = 10,000 m 2
    • In Switzerland, the Ar has the unit name "Are" and is the legal unit for specifying the area of ​​land and parcels.


  • metric carat (ct):
    • As a unit symbol, ct is the legal unit symbol in Switzerland.
    • The use of the intent is not permitted.

Sound level

  • Decibel (dB):
    • Sound pressure level = 20 dB · lg (sound pressure / 20 µPa)
    • Sound power level = 10 dBlg (sound power / 1 pW)
    • Sound intensity level = 10 dBlg (sound intensity / 1 pW / qm)
    • Sound velocity level = 20 dBlg (sound velocity / 50 nm / s)
    • The use of the intent is not permitted.

Other non-SI units

In colloquial but also in technical language usage, some units or unit symbols are used that are not or only partially compliant with the International System of Units. Some examples of such units or symbols are:



Astronomical units of length

  • Astronomical unit
  • Light year (Lj, ly) with light second, light minute, light hour and light day
    • 1 ly = 9 460 730 472 580 800 m
    • The light year is often used in publications addressed to the general public.
  • Parsec (pc)
    • 1 pc ≈ 30.856 776e15 m
    • The parsec is the most frequently used astronomical unit of length in scientific practice. 1 pc is roughly 3.262 light years.


Units in food chemistry

Must weight and sugar content

Original wort content

Acidity of milk and dairy products

Diastatic power of beers

Water hardness


Time and time-related quantities

  • “Auxiliary word / time unit” instead of specifying frequencies in the unit 1 / s or s −1
  • 1 knot (kn) = 1 knot = 1 nautical mile / h = 1.852 km / h ≈ 0.514444 m / s
    • The knot is used as a unit of speed in international aviation and shipping.
  • Radar kilometers (radar mile)
    • 1 radar kilometer = 6.66 µs (1 radar mile = 12.35 µs)
    • The radar kilometer describes the transit time which an electromagnetic wave needs for 1 kilometer ( i.e. for 1 km there and 1 km back), the radar mile 1.852 times the radar kilometer.



  • The use of horsepower ( horsepower ) instead of the legal unit kilowatt (kW) is colloquially widespread especially in motor vehicles.

Dimensionless sizes

  • Further piece dimensions such as B. the dozen = 12 pieces
  • ppb for “parts per billion” and ppt for “parts per trillion” or “parts per thousand”. Since the meaning of the number names billion and trillion is not the same in all languages, the use of these expressions or abbreviations is not recommended.

Individual evidence

  1. See explanation on p. 6 of the SI brochure of the PTB
  2. Le Système international d'unités . 9e édition, 2019 (the so-called "SI brochure", French and English).
  3. The International System of Units (SI) . German translation of the BIPM brochure "Le Système international d'unités / The International System of Units (8e édition, 2006)". In: PTB-Mitteilungen . tape  117 , no. 2 , 2007 ( [PDF; 1.4 MB ] Please note: This is the translation of the SI brochure from 2006. The list of “units outside the SI whose use in connection with the SI is permitted” does not correspond to the status since 2019. The translation of the current version is not yet available in front.).
  4. Appendix 1 of the "Implementing Ordinance to the Law on Units in Metrology and Time Determination (Units Ordinance - EinhV)"
  5. Art. 18 of the Swiss Unit Ordinance
  6. Consolidated version of the guideline (PDF) (as of 2012)
  7. The legal units in Germany, as of June 2015 ( Memento from September 24, 2015 in the Internet Archive )
  8. Foreign exchange rates, euro reference rates, gold, check collection rates. Deutsche Bundesbank, accessed on February 2, 2014 .
  9. Definition in the radar tutorial