Molecular Formula & Weight Calculator
Calculate the molecular weight and elemental composition from a chemical formula, or get the formula and weight from a SMILES string.
Supports brackets and multipliers: (NH4)2SO4, Ca(OH)2
How molecular weight is calculated
The calculator sums the IUPAC standard atomic weights of every atom in the formula. This is the average molecular weight — the value used for concentration calculations (e.g. "dissolve 1 mmol in 1 mL"). The monoisotopic mass used in mass spectrometry is slightly different because it uses the most abundant isotope of each element.
Formula syntax
C6H12O6— glucose(NH4)2SO4— ammonium sulfate, brackets multiplied outCa(OH)2— calcium hydroxide- Element symbols are case sensitive:
Cois cobalt,COis carbon monoxide
Frequently asked questions
Why does the formula change after conversion?
OpenBabel outputs the formula in Hill notation: carbon first, hydrogen
second, then all other elements alphabetically. H8N2O4S and
(NH4)2SO4 describe the same compound.
Does it account for charge or isotopes?
No — the calculation treats every atom as its neutral, naturally abundant isotope.