Rationale The stable HCNOS isotope compositions can be reported as (a) the isotope ratio of two stable isotopes ( R ); (b) the isotope delta value ( δ ); and (c) the atom fraction of the isotopes ( x ). Recalculations between these different expressions are needed frequently and require the use of the absolute isotope ratio for the zero points of the stable isotope delta scales ( R std ). The inconsistent use of R std values may lead to a discrepancy in recalculated results. Methods We summarised the recalculation procedures between different expressions of the stable isotope compositions and introduced a user‐friendly EasyIsoCalculator that allows the recalculation between the main expressions of isotope compositions. We mathematically and empirically evaluated the possible inconsistencies in reporting of the stable isotope data due to the use of different R std and different normalisation methods. Results The recalculation between δ ‐values and other expressions of the stable isotope compositions always involves the use of R std . The choice of R std will have a significant influence on the recalculated values. The use of different R std values has a significant influence also on the normalisation of raw values but only when the normalisation is conducted versus the working standard gas value, causing discrepancy, e.g. for δ ( 13 C/ 12 C) up to ~ 0.3 ‰. Conclusions Differences in the selection of R std value may lead to significant differences among different laboratories. The uncertainty in the calculations originates primarily from the uncertainty in the R std determination; however, it is lower than the discrepancy arising from the inconsistent use of R std . Consistent use of the same R std values is required to eliminate the unnecessary discrepancy if different data sets are recalculated from delta value to other expressions.