As a result of international agreements a unification of the measurement system has been achieved (International System of Units; Système international = SI-units). In Germany the “Law on Units in the Measurement System” was passed on 2 July 1969, the implementing ordinance concerning this Law was passed on 26 July 1970 and a Law for amending the Law on Units in the Measurement System on 6 July 1973. According to this legal framework the unitsdefined in these laws and regulations are obligatory for business and official transactions. Details have been defined in DIN 1301 “Units, unit names, unit symbols” (DIN Deutsches Institut für Normung e. V., Burggrafenstraße 6, 10787 Berlin, Tel.: +49 30 2601-0, Fax: +49 30 2601-1231 and in ISO 31, which is now superseded by the harmonized ISO/IEC 80000 standard (ISO copyright office, Case postale 56, CH-1211 Geneva 20, Tel. + 41 22 749 01 11, Fax + 41 22 749 09 47.

In the USA the same units etc. were first described in The Metric Conversion Act of 1975 (Public Law 94–168) (see also G.G. Stoner, Cosmet. Toiletries 93, No. 11, 57 [1978]) and are now harmonized in ISO 80000.

The new units have been taken into account in the present edition of the Handbook, when new documentation has been provided by the manufacturers of auxiliary substances. The new units have also been used in some revisions and additions by the editors. However, in the following the most important changes in the measurement system are given so that conversions from older unit systems may be performed, if required.

There are nine basic SI units, of which the following 7 are relevant:

SI-Basic Units

Basis physical quantity	Basic unit
	Name	Symbol
length	meter	m
time	second	s
mass	kilogram	kg
temperature	kelvin	K
electrical current	ampere	A
luminous intensity	candela	cd
amount of substance	mole	mol

17 further units are derived from the nine basic units, of which the following derived SI units, with their names,are relevant:

Physical quantity	Unit		Expressed as basic units or derived units
	Name	Symbol
work, energy, quantity of heat	joule	J
illuminance	lux	lx
force	Newton	N
pressure	Pascal	Pa

The following prefixes were established for decimal multiples and subdivisions of units:

Multiples	Prefix	Symbol	Subdivisions	Prefix	Symbol
101	deca	da	10–1	deci	d
102	hecto	h	10–2	centi	c
103	kilo	k	10–3	milli	m
106	mega	M	10–6	micro	µ
109	giga	G	10–9	nano	n
1012	tera	T	10–12	pico	p
1015	peta	P	10–15	femto	f
1018	exa	E	10–18	atto	a

Legal Units

Physical quantity	Legal units				Conversion		Comment
	Symbol	SI Unit		Relationship	Recommended unit	Previous unit
		Name	Symbol
mass	m	kilogram	kg	;	kg		SI basic unit
density	ρ	kilogram per cubic meter			;
specific volume	V	cubic meter per kilogram			;
amount of substance	n	mole	mol		kmol
mass related to amount of substance (molar mass)	M	kilogram per mol
volume related to amount of substanc (molar volume)	Vm	cubic meter per mole
surface tension and interfacial tension		milli-Newton per meter					for all liquids
thermodynamic or kelvin temperature, celsius temperature	T, Θ, t, ϑ	Kelvin	K		K ℃	°K	SI basic unit practical conversion:
dynamic viscosity	η	Pascal second	Pa·s		mPa·s	cP (centipoise)
kinematic viscosity	ν	square meter per second				st (stokes)