Tools
Molecular weight calculator
Paste a SMILES string to get average molecular weight, monoisotopic mass, formula, and atom count — instantly, in your browser.
Result
- Molecular formula
- C9H8O4
- Average molecular weight
- 180.16 g/mol
- Monoisotopic mass
- 180.0423 g/mol
- Heavy atoms
- 13
This calculator reads a SMILES string, builds the molecule locally, and reports its molecular formula, average molecular weight, and monoisotopic mass. The average weight uses standard atomic weights and is what you use for stoichiometry; the monoisotopic mass uses the most abundant isotope of each element and is what mass spectrometry reports. Nothing is uploaded — the computation runs entirely in your browser.
How to calculate molecular weight from a SMILES string
- 1.Paste or type the SMILES string of your molecule into the input box.
- 2.The molecule is parsed instantly and its 2D structure is drawn so you can confirm it is correct.
- 3.Read the molecular formula, average molecular weight (g/mol), and monoisotopic mass from the result panel.
- 4.Use the average weight for stoichiometry and the monoisotopic mass when interpreting mass spectra.
Frequently asked questions
- What is the difference between average and monoisotopic mass?
- Average molecular weight is calculated from the standard atomic weights (a natural-abundance average of isotopes) and is used for stoichiometry. Monoisotopic mass uses the single most abundant isotope of each element and is what high-resolution mass spectrometry measures.
- Does this tool send my structure to a server?
- No. The molecule is parsed and all values are computed locally in your browser using openchemlib. Your SMILES string never leaves your device.
- What input format does the calculator accept?
- It accepts a SMILES string, for example CC(=O)Oc1ccccc1C(=O)O for aspirin. If the string cannot be parsed, the tool tells you instead of returning a wrong number.