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.
| Author: | Christoph Gohlke |
|---|---|
| License: | BSD-3-Clause |
| Version: | 2026.1.8 |
| DOI: | 10.5281/zenodo.7135495 |
Install the molmass package and all dependencies from the Python Package Index:
python -m pip install -U "molmass[all]"
Print the console script usage:
python -m molmass --help
Run the web application:
python -m molmass --web
The molmass library is documented via docstrings.
See Examples for using the programming interface.
Source code and support are available on GitHub.
This revision was tested with the following requirements and dependencies (other versions may work):
- CPython 3.11.9, 3.12.10, 3.13.11, 3.14.2
- Flask 3.1.2 (optional)
- Pandas 2.3.3 (optional)
- wxPython 4.2.4 (optional)
2026.1.8
- Improve code quality.
2025.12.12
- Make boolean arguments keyword-only (breaking).
2025.11.11
- Allow empty formulas (breaking).
- Derive FormulaError from ValueError.
- Move tests to separate test module.
2025.9.4
- Precompile regex patterns.
- Remove doctest command line option.
- Drop support for Python 3.10, support Python 3.14.
2025.4.14
- Add mass_charge_ratio helper function (#17).
- Drop support for Python 3.9.
2024.10.25
- …
Refer to the CHANGES file for older revisions.
Calculate the molecular mass, elemental composition, and mass distribution of a molecule from its chemical formula:
>>> from molmass import Formula
>>> f = Formula('C8H10N4O2') # Caffeine
>>> f
Formula('C8H10N4O2')
>>> f.formula # hill notation
'C8H10N4O2'
>>> f.empirical
'C4H5N2O'
>>> f.mass # average mass
194.1909...
>>> f.nominal_mass # == f.isotope.massnumber
194
>>> f.monoisotopic_mass # == f.isotope.mass
194.0803...
>>> f.atoms
24
>>> f.charge
0
>>> f.composition().dataframe()
Count Relative mass Fraction
Element...
C 8 96.085920 0.494801
H 10 10.079410 0.051905
N 4 56.026812 0.288514
O 2 31.998810 0.164780
>>> f.spectrum(min_intensity=0.01).dataframe()
Relative mass Fraction Intensity % m/z
Mass number...
194 194.080376 0.898828 100.000000 194.080376
195 195.082873 0.092625 10.305100 195.082873
196 196.084968 0.008022 0.892492 196.084968
197 197.087214 0.000500 0.055681 197.087214Access physicochemical and descriptive properties of the chemical elements:
>>> from molmass import ELEMENTS, Element
>>> e = ELEMENTS['C']
>>> e
Element(
6, 'C', 'Carbon',
group=14, period=2, block='p', series=1,
mass=12.01074, eleneg=2.55, eleaffin=1.262118,
covrad=0.77, atmrad=0.91, vdwrad=1.7,
tboil=5100.0, tmelt=3825.0, density=3.51,
eleconfig='[He] 2s2 2p2',
oxistates='4*, 2, -4*',
ionenergy=(
11.2603, 24.383, 47.877, 64.492, 392.077,
489.981,
),
isotopes={
12: Isotope(12.0, 0.9893, 12),
13: Isotope(13.00335483507, 0.0107, 13),
},
)
>>> e.number
6
>>> e.symbol
'C'
>>> e.name
'Carbon'
>>> e.description
'Carbon is a member of group 14 of the periodic table...'
>>> e.eleconfig
'[He] 2s2 2p2'
>>> e.eleconfig_dict
{(1, 's'): 2, (2, 's'): 2, (2, 'p'): 2}
>>> str(ELEMENTS[6])
'Carbon'
>>> len(ELEMENTS)
109
>>> sum(e.mass for e in ELEMENTS)
14693.181589001...
>>> for e in ELEMENTS:
... e.validate()
...