The present invention provides a method for the determination of the monoisotopic mass of a macromolecule from a mass Mmono spectrometry spectrum of said macromolecule based on the experimentally determined most abundant mass, with accuracy in the low parts-per-million (ppm) range. The method uses a simple, double-linear model for predicting the monoisotopic mass based on the experimentally determined most abundant mass, comprising the steps of (a) deriving the most abundant mass MMostAb from the spectrum; and (b) calculating the monoisotopic mass MMono from the most abundant mass MMostAb, using MMono=a+βMMOSTAB+ε; wherein β is a scalar slope obtainable by fitting the slope of monoisotopic mass versus most abundant mass for a plurality of macromolecules from a macromolecule database; and ε is a scalar residue of the form ε=εint+sfrac, εint being an integer, and εfrac being a sawtooth function of MMostAb.