Molmass is a Python library, console script, and web application to calculate the molecular mass (average, nominal, and isotopic pure), the elemental composition, and the mass distribution spectrum of a molecule given by its chemical formula, relative element weights, or sequence. Calculations are based on the isotopic composition of the elements. Mass deficiency due to chemical bonding is not taken into account. The library includes a database of physicochemical and descriptive properties of the chemical elements.