Molar Mass Calculator
Molar mass from a chemical formula — parentheses, brackets, hydrates (CuSO4·5H2O), coefficients — with each element’s count, mass and mass %, and grams ↔ moles.
The molar mass in g/mol, the element breakdown with mass percent, the atom count, moles for a mass or grams for an amount, and a flag for elements without a standard atomic weight.
Example: CuSO4·5H2O is 249.68 g/mol (O 57.7 %, Cu 25.5 %, S 12.8 %, H 4.0 %); 25 g is 0.1001 mol. Ca(OH)2 is 74.09 g/mol; [Fe(CN)6]3− is 211.95 with the charge ignored.
Count the atoms,
weigh them.
How the formula is read, where the atomic weights come from, and what the flags mean.
Reading a formula
Element symbols with counts (H2O), parentheses and brackets with a multiplier (Ca(OH)2, [Fe(CN)6]), hydrate dots with a coefficient (CuSO4·5H2O; "." and "*" also work), a leading coefficient (2NaCl), Unicode subscripts, and a trailing charge (3−, 2+) which is ignored because an electron weighs next to nothing. Anything that is not an element symbol stops the parse with a message.
The weights
Molar mass = Σ count × atomic weight, using the IUPAC abridged standard atomic weights (five significant figures, as printed on most periodic tables). For elements with no stable isotope and no standard weight — Tc, Pm, Po and beyond — the mass number of the longest-lived isotope stands in and the result is flagged as approximate. Mass percent = element mass ÷ molar mass.
Not handled
Isotopic labels (D, ¹³C), non-stoichiometric notation with x, ionic charges affecting mass, and the exact-value weights for high-precision work (use the full IUPAC intervals for that). Moles = grams ÷ molar mass and grams = moles × molar mass are given for one amount. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- IUPAC CIAAW abridged standard atomic weights (2021, five significant figures); elements without a standard weight use the mass number of the longest-lived isotope and are flagged
Last reviewed 21 September 2026. How results are checked: How we verify.