POTAMOS mass spectrometry calculator, an easy to use, free proteomics web tool is developed in the open source application framework Ruby on Rails. It is designed to provide calculated mass spectrometry data and aid to the experimental biochemist/biologist who works on the field of post-translational modifications of proteins while it focuses especially on histone proteins. The main calculation facilities provided to the experimental biochemist/biologist by POTAMOS mass spectrometry calculator are: 1. Calculation of the masses of either a given unmodified amino acid sequence (AAS) or that of an AAS that can potentially carry methylations of arginines and lysines, acetylations of lysines or phosphorylations of serines and threonines. 2. Calculation of the masses and charges of a fragmented histone variant, which carries one, or more predefined modifications of the types described above. 3. Calculation of the masses of peptide fragments produced through the action of a protease on a histone.