symbols, acronyms & abbreviations



parts per million: usually accompanied by an indication of the property being measured, often “(wt)”, weight, usually actually mass, or “(vol)”, volume.

Use of this term is strongly discouraged by NIST in favor of the actual units appropriate to the property and magnitude, for example, “ppm (vol)” can be expressed as microliters per liter.


In keeping with Ref. [6: ISO 31-0], this Guide takes the position that the language-dependent terms part per million, part per billion and part per trillion, and their respective abbreviations ‘ppm’, ‘ppb’ and ‘ppt’ (and similar terms and abbreviations), are not acceptable for use with the SI to express the values of quantities. Forms such as those given in the following examples should be used instead.


a stability of 0.5 (mA/A)/min but not: a stability of 0.5 ppm/min
a shift of 1.1 nm/m but not: a shift of 1.1 ppb
a frequency change of 0.35 × 1029 f but not: a frequency change of 0.35 ppb
a sensitivity of 2 ng/kg but not: a sensitivity of 2 ppt

the relative expanded uncertainty of the resistance R is Ur = 3 mV/V
the expanded uncertainty of the resistance R is U = 3 × 10−6 R
the relative expanded uncertainty of the resistance R is Ur = 3 × 10−6

but not: the relative expanded uncertainty of the resistance R is Ur = 3 ppm

Because the names of numbers 109 and larger are not uniform worldwide, it is best that they be avoided entirely (in most countries, 1 billion = 1 × 1012, not 1 × 109 as in the United States); the preferred way of expressing large numbers is to use powers of 10. This ambiguity in the names of numbers is one of the reasons why the use of ppm, ppb, ppt, and the like is deprecated. Another, and a more important one, is that it is inappropriate to use abbreviations that are language dependent together with internationally recognized signs and symbols, such as MPa, ln, 1013, and %, to express the values of quantities and in equations or other mathematical expressions (see also Sec. 7.6).

Note : This Guide recognizes that in certain cases the use of ppm, ppb, and the like may be required by a law or a regulation. Under these circumstances, Secs. 2.1 and 2.1.1 apply.”

Barry N. Taylor.
Guide for the Use of the International System of Units (SI)
NIST Special Publication 811, 1995 edition.
Washington, DC: U.S.G.P.O., 1995.
Section 7.10.3.


picks per minute: a measure used to describe the speed of looms.

Wall text, American Textile History Museum, Lowell (MA), 2005.

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