Parts-per notation

Last updated on 29 April 2017

In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement. Commonly used are ppm (parts-per-million, 10−6), ppb (parts-per-billion, 10−9), ppt (parts-per-trillion, 10−12) and ppq (parts-per-quadrillion, 10−15).

Fluorescein (2).jpg
Fluorescein aqueous solutions, diluted from 1 — 10000 parts-per-million


Parts-per notation is often used describing dilute solutions in chemistry, for instance, the relative abundance of dissolved minerals or pollutants in water. The unit “1 ppm” can be used for a mass fraction if a water-borne pollutant is present at one-millionth of a gram per gram of sample solution. When working with aqueous solutions, it is common to assume that the density of water is 1.00 g/mL. Therefore, it is common to equate 1 kilogram of water with 1 L of water. Consequently, 1 ppm corresponds to 1 mg/L and 1 ppb corresponds to 1 μg/L.

Similarly, parts-per notation is used also in physics and engineering to express the value of various proportional phenomena. For instance, a special metal alloy might expand 1.2 micrometers per meter of length for every degree Celsius and this would be expressed as “α = 1.2 ppm/°C.” Parts-per notation is also employed to denote the change, stability, or uncertainty in measurements. For instance, the accuracy of land-survey distance measurements when using a laser rangefinder might be 1 millimeter per kilometer of distance; this could be expressed as “Accuracy = 1 ppm.”

Parts-per notations are all dimensionless quantities: in mathematical expressions, the units of measurement always cancel. In fractions like “2 nanometers per meter” (2 nm/m = 2 nano = 2 × 10−9 = 2 ppb = 2 × 0.000000001) so the quotients are pure-number coefficients with positive values less than 1. When parts-per notations, including the percent symbol (%), are used in regular prose (as opposed to mathematical expressions), they are still pure-number dimensionless quantities. However, they generally take the literal “parts per” meaning of a comparative ratio (e.g., “2 ppb” would generally be interpreted as “two parts in a billion parts”).

Parts-per notations may be expressed in terms of any unit of the same measure. For instance, the coefficient of thermal expansion of a certain brass alloy, α = 18.7 ppm/°C, may be expressed as 18.7 (µm/m)/°C, or as 18.7 (µin/in)/°C; the numeric value representing a relative proportion does not change with the adoption of a different unit of measure. Similarly, a metering pump that injects a trace chemical into the main process line at the proportional flow rate Qp = 125 ppm, is doing so at a rate that may be expressed in a variety of volumetric units, including 125 µL/L, 125 µgal/gal, 125 cm3/m3, etc.

In nuclear magnetic resonance (NMR) spectroscopy

In nuclear magnetic resonance spectroscopy (NMR), chemical shift is usually expressed in ppm. It represents the difference of a measured frequency in parts per million from the reference frequency. The reference frequency depends on the instrument's magnetic field and the element being measured. It is usually expressed in MHz. Typical chemical shifts are rarely more than a few hundred Hz from the reference frequency, so chemical shifts are conveniently expressed in ppm (Hz/MHz). Parts-per notation gives a dimensionless quantity that does not depend on the instrument's field strength.

Parts-per expressions

Visualisation parts per.svg
Visualisation of 1%, 1‰, 1‱ and 1 ppm as the fraction of the red cube to its respective block (click for larger version)
  • One part per hundred is generally represented by the percent (%) symbol and denotes one part per 100 parts, one part in 102, and a value of 1 × 10−2. This is equivalent to approximately one 'drop' (circa 1/20mL) of water diluted into 5 milliliters (one teaspoonful) or about fifteen minutes out of one day.
  • One part per thousand should generally be spelled out in full and not as "ppt" (which is usually understood to represent "parts per trillion"). It may also be denoted by the millage () symbol. Note however, that specific disciplines such as oceanography, as well as educational exercises, do use the "ppt" abbreviation. "One part per thousand" denotes one part per 1000 parts, one part in 103, and a value of 1 × 10−3. This is equivalent to one drop of water diluted into 50 milliliters (ten spoon-fulls) or about one and a half minutes out of one day.
  • One part per ten thousand is denoted by the permyriad (‱) symbol. In contrast, in finance, the basis point is a quantity with dimensions of (time−1) and is typically used to denote changes in or differences between percentage interest rates. For instance, a change in an interest rate from 5.15% per annum to 5.35% per annum could be denoted as a change of 20 basis points. Although rarely used in science (ppm is typically used instead), one permyriad has an unambiguous value of one part per 10000 parts, one part in 104, and a value of 1 × 10−4. This is equivalent to one drop of water diluted into half a liter or about nine seconds out of one day.
  • One part per million (ppm) denotes one part per 1000000 parts, one part in 106, 1/1000000 × 100% = 0.0001% (or 1% = 10000 ppm), and a value of 1 × 10−6. This is equivalent to one drop of water diluted into 50 liters (roughly the fuel tank capacity of a compact car) or about 32 seconds out of a year; in distance measurement, it is equivalent to 1 mm of error per km of distance traversed.
  • One part per billion (ppb) denotes one part per 1000000000 parts, one part in 109, 1/1000000000 × 100% = 0.0000001% (or 1% = 10000000 ppb) and a value of 1 × 10−9. This is equivalent to one drop of water diluted into 250 chemical drums (50 m3), or about three seconds out of a century.
  • One part per trillion (ppt) denotes one part per 1000000000000 parts, one part in 1012, and a value of 1 × 10−12. This is equivalent to one drop of water (0.05 mL) diluted into 20 Olympic-size swimming pools (2500 m3), or about three seconds out of every hundred thousand years.
  • One part per quadrillion (ppq) denotes one part per 1000000000000000 parts, one part in 1015, and a value of 1 × 10−15. This is equivalent to 1 drop of water diluted into a cube of water measuring approximately 368 meters on a side (fifty million cubic meters, which is a cube about as tall as the Empire State Building's 102 stories), or two and a half minutes out of the age of the Earth (4.5 billion years). Although relatively uncommon in analytical chemistry, measurements at the ppq level are sometimes performed.


Although the International Bureau of Weights and Measures (an international standards organization known also by its French-language initials BIPM) recognizes the use of parts-per notation, it is not formally part of the International System of Units (SI). Note that although “percent” (%) is not formally part of the SI, both the BIPM and the ISO take the position that “in mathematical expressions, the internationally recognized symbol % (percent) may be used with the SI to represent the number 0.01” for dimensionless quantities. According to IUPAP, “a continued source of annoyance to unit purists has been the continued use of percent, ppm, ppb, and ppt.”. Although SI-compliant expressions should be used as an alternative, the parts-per notation remains nevertheless widely used in technical disciplines. The main problems with the parts-per notation are the following:

Long and short scales

Because the named numbers starting with a “billion” have different values in different countries, the BIPM suggests avoiding the use of “ppb” and “ppt” to prevent misunderstanding. In the English language, named numbers have a consistent meaning only up to “million”. Starting with “billion”, there are two numbering conventions: the “long” and “short” scales, and “billion” can mean either 1012 or 109. The U.S. National Institute of Standards and Technology (NIST) takes the stringent position, stating that “the language-dependent terms [ . . . ] are not acceptable for use with the SI to express the values of quantities.”

Thousand vs. trillion

Although "ppt" usually means "parts per trillion", it occasionally means "parts per thousand". Unless the meaning of "ppt" is defined explicitly, it has to be guessed from the context.

Mass fraction vs. mole fraction vs. volume fraction

Another problem of the parts-per notation is that it may refer to mass fraction, mole fraction or volume fraction. Since it is usually not stated which quantity is used, it is better to write the unit as kg/kg, mol/mol or m3/m3 (even though they are all dimensionless). The difference is quite significant when dealing with gases and it is very important to specify which quantity is being used. For example, the conversion factor between a mass fraction of 1 ppb and a mole fraction of 1 ppb is about 4.7 for the greenhouse gas CFC-11 in air. For volume fraction, the suffix "V" or "v" is sometimes appended to the parts-per notation (e.g., ppmV, ppbv, pptv). Unfortunately, ppbv and pptv are also often used for mole fractions (which is identical to volume fraction only for ideal gases).

The usage is generally quite fixed inside most specific branches of science, leading some researchers to draw the conclusion that their own usage (mass/mass, mol/mol, volume/volume, or others) is the only correct one. This, in turn, leads them to not specify their usage in their publications, and others may therefore misinterpret their results. For example, electrochemists often use volume/volume, while chemical engineers may use mass/mass as well as volume/volume. Many academic papers of otherwise excellent level fail to specify their usage of the parts-per notation.

SI-compliant expressions

SI-compliant units that can be used as alternatives are shown in the chart below. Expressions that the BIPM does not explicitly recognize as being suitable for denoting dimensionless quantities with the SI are shown in underlined green text.

Notations for dimensionless quantities
Measure SI
parts-per ratio
(short scale)
or symbol
Value in
A strain of… 2 cm/m 2 parts per hundred     2% 2 × 10−2
A sensitivity of… 2 mV/V 2 parts per thousand 2 ‰ 2 × 10−3
A sensitivity of… 0.2 mV/V 2 parts per ten thousand 2 ‱ 2 × 10−4
A sensitivity of… 2 µV/V 2 parts per million 2 ppm 2 × 10−6
A sensitivity of… 2 nV/V 2 parts per billion 2 ppb 2 × 10−9
A sensitivity of… 2 pV/V 2 parts per trillion 2 ppt 2 × 10−12
A mass fraction of… 2 mg/kg 2 parts per million 2 ppm 2 × 10−6
A mass fraction of… 2 µg/kg 2 parts per billion 2 ppb 2 × 10−9
A mass fraction of… 2 ng/kg 2 parts per trillion 2 ppt 2 × 10−12
A mass fraction of… 2 pg/kg 2 parts per quadrillion 2 ppq 2 × 10−15
A volume fraction of… 5.2 µL/L 5.2 parts per million 5.2 ppm 5.2 × 10−6
A mole fraction of… 5.24 µmol/mol 5.24 parts per million 5.24 ppm 5.24 × 10−6
A mole fraction of… 5.24 nmol/mol 5.24 parts per billion 5.24 ppb 5.24 × 10−9
A mole fraction of… 5.24 pmol/mol 5.24 parts per trillion 5.24 ppt 5.24 × 10−12
A stability of… 1 (µA/A)/min. 1 part per million per min. 1 ppm/min. 1 × 10−6/min.
A change of… 5 nΩ/Ω 5 parts per billion 5 ppb 5 × 10−9
An uncertainty of… 9 µg/kg 9 parts per billion 9 ppb 9 × 10−9
A shift of… 1 nm/m 1 part per billion 1 ppb 1 × 10−9
A strain of… 1 µm/m 1 part per million 1 ppm 1 × 10−6
A temperature coefficient of… 0.3 (µHz/Hz)/°C 0.3 part per million per °C 0.3 ppm/°C 0.3 × 10−6/°C
A frequency change of… 0.35 × 10−9 ƒ 0.35 part per billion 0.35 ppb 0.35 × 10−9

Note that the notations in the “SI units” column above are all dimensionless quantities; that is, the units of measurement factor out in expressions like “1 nm/m” (1 nm/m = 1 nano = 1 × 10−9) so the quotients are pure-number coefficients with values less than 1.


Because of the cumbersome nature of expressing certain dimensionless quantities per SI guidelines, the International Union of Pure and Applied Physics (IUPAP) in 1999 proposed the adoption of the special name "uno" (symbol: U) to represent the number 1 in dimensionless quantities. This symbol is not to be confused with the always-italicized symbol for the variable "uncertainty" (symbol: U). This unit name "uno" and its symbol could be used in combination with the SI prefixes to express the values of dimensionless quantities that are much less—or even greater—than one.

Common parts-per notations in terms of the uno are given in the table below.

IUPAP's "uno" proposal
Coefficient Parts-per example Uno equiv. Symbol form Value of quantity
10−2 2% 2 centiuno 2 cU 2 × 10−2
10−3 2 ‰ 2 milliuno 2 mU 2 × 10−3
10−4 2 ‱ 0.2 milliuno 0.2 mU 2 × 10−4
10−6 2 ppm 2 microuno 2 µU 2 × 10−6
10−9 2 ppb 2 nanouno 2 nU 2 × 10−9
10−12 2 ppt 2 picouno 2 pU 2 × 10−12

In 2004, a report to the International Committee for Weights and Measures (known also by its French-language initials CIPM) stated that response to the proposal of the uno "had been almost entirely negative" and the principal proponent "recommended dropping the idea". To date, the uno has not been adopted by any standards organization and it appears unlikely it will ever become an officially sanctioned way to express low-value (high-ratio) dimensionless quantities. The proposal was instructive, however, as to the perceived shortcomings of the current options for denoting dimensionless quantities.

Improper applications

Parts-per notation may properly be used only to express true dimensionless quantities; that is, the units of measurement must cancel in expressions like "1 mg/kg" so that the quotients are pure numbers with values less than 1. Mixed-unit quantities such as "a radon concentration of 15 pCi/L" are not dimensionless quantities and may not be expressed using any form of parts-per notation, such as "15 ppt". Other examples of measures that are not dimensionless quantities are as follows:

  • Particulate matter in the air: 50 µg/m3; not 50 ppb.
  • A stepper motor/gear system that produces a motion of 1 µm/pulse; not 1 ppm
  • Mercury vapor concentration in air: 0.6 ng/L; not 0.6 ppt

Note however, that it is not uncommon to express aqueous concentrations—particularly in drinking-water reports intended for the general public—using parts-per notation (2.1 ppm, 0.8 ppb, etc.) and further, for those reports to state that the notations denote milligrams per liter or micrograms per liter. Although "2.1 mg/L" is not a dimensionless quantity, it is assumed in scientific circles that "2.1 mg/kg" (2.1 ppm) is the true measure because one liter of water has a mass of about one kilogram. The goal in all technical writing (including drinking-water reports for the general public) is to clearly communicate to the intended audience with minimal confusion. Drinking water is intuitively a volumetric quantity in the public’s mind so measures of contamination expressed on a per-liter basis are considered to be easier to grasp. Still, it is technically possible, for example, to "dissolve" more than one liter of a very hydrophilic chemical in 1 liter of water; parts-per notation would be confusing when describing its solubility in water (greater than a million parts per million), so one would simply state the volume (or mass) that will dissolve into a liter, instead.

When reporting air-borne rather than water-borne densities, a slightly different convention is used since air is approximately 1000 times less dense than water. In water, 1 µg/m3 is roughly equivalent to parts-per-trillion whereas in air, it is roughly equivalent to parts-per-billion. Note also, that in the case of air, this convention is much less accurate. Whereas one liter of water is almost exactly 1 kg, one cubic meter of air is often taken as 1.143 kg—much less accurate, but still close enough for many practical uses.

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