The NIST Isotope Enrichment Calculator is a program written in Visual Basic which determines the percentage of 15N enrichment of stable isotope-labeled peptides or proteins. A multinomial analysis of Hsu (Anal. Chem. 560 1356-1361 (1984)) is used to determine the theoretical isotopic distribution of an input sequence at incremental isotopic percentages which are compared with an input experimentally derived profile. The theoretical profile of 15N percentage which maximizes the Pearson correlation as described by MacCoss et al. (Anal. Chem. 77 7646-7653 (2005)) is output along with the coefficient.