Mass Spec m/z Ratio Calculator

About this calculator

In mass spectrometry, the observed mass-to-charge ratio for a protonated ion is m/z = (M + n × 1.00728) / n, where M is the neutral molecule's mass, n is the charge state (number of protons added), and 1.00728 is the mass of a proton in Daltons. A singly-charged ion (n=1) shows m/z close to M+1; a doubly-charged ion (n=2) shows roughly half that.

Analytical and biological chemists use this to predict where a compound's peaks will appear on a mass spectrum before running a sample, and to work backward from an observed peak to identify a molecule's neutral mass when the charge state is known or can be inferred from isotope spacing.