The relative atomic mass (the term atomic weight was previously used) given in the table below is scaled relatively to the isotope ¹²C taken as 12 exactly and is given to 4 significant figures. Only the elements most commonly encountered in pharmaceutical analysis are included.

Most elements have more than one naturally occurring isotope and the variation in the relative abundance of these isotopes influences the precision with which the relative atomic mass of an element in nature can be quoted. Among the elements included in the table, the relative atomic mass of samples in nature of boron, calcium, lead, strontium and sulfur may differ by more than one in the fourth significant figure.

* Values are considered reliable to ± 1 in the last digit or ± 3 when followed by an asterisk.