Project Details IUPAC Molecular Weight Calculator

Project No.:
Start Date:
15 November 2015
End Date:


Through the Commission on Isotopic Abundances and Atomic Weights (CIAAW), IUPAC provides authoritative atomic weights and this project sets to ensure that IUPAC also takes an action in providing quantities, which are derived from these atomic weights. These are: molecular weights, isotopic abundances and isotope ratios of molecules (isotope patterns). Plenty of molecular weight calculators are available online to offer this information; yet, none of them incorporate uncertainty estimates for either the molecular weights or the corresponding isotope ratios and isotopic abundances. In addition, many of the available calculators use outdated or incorrect input data.

It is the objective of this project to create a web-based calculator, hosted on, with molecular formula as the input parameter. This will later be also modified such that a common name can be entered and elemental ratios will be added as a possible output.


Using a web based interface, the user will be able to define the atomic composition of any specific molecule. The IUPAC Molecular Weight Calculator will then provide information about the molecular weight, isotopic abundances and isotope ratios of molecules along with the uncertainties and correlations between all output variables.

This information (elemental isotope ratios, uncertainties and covariances) is currently unavailable from CIAAW reports and users have to perform their own calculations. Performing such calculations is not trivial and this calculator will utilize recent developments in pure and applied mathematics to enable fast isotope pattern calculations, which are needed to establish the uncertainties. For example, one cannot simply employ binomial expansion to obtain isotopic abundances of molecules as such naïve approach will not enable to perform Monte-Carlo calculations in order to obtain the necessary uncertainty distributions. This calculator will take advantage of recent developments in applied mathematics and will employ either Fast Fourier Transformations or Newton-Girard equations to enable fast calculations.

In addition, instead of dealing with standard atomic weights, users will also be able to specify specific sources for the elements of the molecules (indicated in CIAAW SNIF diagrams) or specify isotope delta values for these elements. Given that some technical details of the isotope pattern calculations are rather novel, they will be disclosed in technical peer-reviewed publications.


In September 2016, a stable beta version was launched on ( and communicated to CIAAW members and Division II representatives for input. The calculator currently focuses on the uncertainty calculations using fast Fourier Transforms mathematics and it provides molecular weight, isotopic abundances and isotope ratios from the given compositional formula (without parenthesis), including the correlations between the isotope ratios. Further functionality will be added, most notably the ability to enter user-specified isotopic compositions of the elements and the ability to specify isotopic composition as delta value.

Dec 2020 update – Efforts are underway to host the calculator on server and release it publicly.
A formal publication and announcement of the calculator is expected in 2021.

Aug 2021 update – A functioning prototype calculator has been deployed at A final version will be released soon.

Page last update 16 Aug 2021