Editing Parts-per notation (section)

{{Short description|Set of units to describe small values}}
{{Redirect|Parts per billion|the film|Parts per Billion{{!}}''Parts per Billion''}}
[[File:Fluorescein (2).jpg|thumb|upright|[[Fluorescein]] aqueous solutions, diluted from {{val|10000}} to 1{{nbsp}}part per million in intervals of ten-fold dilution.

At {{val|10000|u=ppm}} the solution is a deep red colour. As the concentration decreases the colour becomes orange, then a vibrant yellow, with the final 1{{nbsp}}ppm sample a very pale yellow.]]

In [[science]] and [[engineering]], the '''parts-per notation''' is a set of pseudo-units to describe the small values of miscellaneous [[dimensionless quantity|dimensionless quantities]], e.g. [[mole fraction]] or [[mass fraction (chemistry)|mass fraction]]. 

Since these [[fraction (mathematics)|fractions]] are quantity-per-quantity measures, they are pure numbers with no associated [[units of measurement]]. Commonly used are

* '''parts-per-million''' '''- ppm''', {{nowrap|10<sup>−6</sup>}}
* '''parts-per-billion''' '''- ppb''', {{nowrap|10<sup>−9</sup>}}
* '''parts-per-trillion''' '''- ppt''', {{nowrap|10<sup>−12</sup>}}
* '''parts-per-quadrillion''' '''- ppq''', {{nowrap|10<sup>−15</sup>}} 

This notation is not part of the [[International System of Units]] - SI system and its meaning is ambiguous.