Of the 25 known isotopes of sulfur, four are stable. In order of their abundance, those isotopes are S (94.93%), S (4.29%), S (0.76%), and S (0.02%). The δS value refers to a measure of the ratio of the two most common stable sulfur isotopes, S:S, as measured in a sample against that same ratio as measured in a known reference standard. The lowercase delta character is used by convention, to be consistent with use in other areas of stable isotope chemistry. That value can be calculated in per mil (‰, parts per thousand) as:
Less commonly, if the appropriate isotope abundances are measured, similar formulae can be used to quantify ratio variations between S and S, and S and S, reported as δS and δS, respectively.