A primary standard in metrology is a standard that is sufficiently accurate such that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass and time. Primary standards are used to calibrate other standards referred to as working standards. See Hierarchy of Standards.

In chemistry

Standards are used in analytical chemistry. Here, a primary standard is typically a reagent which can be weighed easily, and which is so pure that its weight is truly representative of the number of moles of substance contained. Features of a primary standard include: (The last two are not as essential as the first four.) Some examples of primary standards for titration of solutions, based on their high purity, are provided: Such standards are often used to make standard solutions. These primary standards are used in titration and are essential for determining unknown concentrations or preparing working standards.