Molecular Weight Calculator
Enter a chemical formula and get molar mass, exact mass, Hill notation, atom counts, and per-element composition.
About this tool
Use this molecular weight calculator to turn a chemical formula into molar mass, monoisotopic mass, Hill notation, atom counts, and a per-element composition table. It is designed for formula checks in the browser: no upload, account, or network lookup is needed after the page loads.
Worked example: C8H10N4O2 (caffeine) reports a molar mass of about
194.194 g/mol, a monoisotopic mass of about 194.0804 Da, 24 total atoms, and
composition rows for carbon, hydrogen, nitrogen, and oxygen.
The parser supports normal element symbols and counts (H2O, NaCl,
C6H12O6), nested groups with multipliers (Ca(OH)2, K4[Fe(CN)6]), and
hydrate dot notation with a leading coefficient (CuSO4·5H2O, CuSO4.5H2O,
or Na2CO3*10H2O). Element symbols are case-sensitive, so Co means cobalt
while CO means carbon plus oxygen.
Limits and edge cases: formulas must use element symbols H through U from the embedded atomic-mass table. Isotopic labels, charges, structural formulas, fractional stoichiometry, and SMILES strings are not parsed. The exact mass is the sum of each element's most abundant isotope; it is useful for quick checks, not a substitute for a full isotope-pattern engine.
FAQ
Does this accept SMILES strings?
No. SMILES parsing needs a chemistry graph engine and is outside this pure
formula parser. Enter the molecular formula instead, such as C6H6 for benzene
or C8H10N4O2 for caffeine.
What is the difference between molar mass and monoisotopic mass?
Molar mass uses standard atomic weights and is reported in g/mol. The
monoisotopic mass uses the exact mass of the most abundant isotope for each
element and is reported in daltons (Da).
Can I calculate hydrates and grouped formulas?
Yes. Use parentheses or brackets for groups, such as Ca(OH)2, and dot notation
for hydrates, such as CuSO4·5H2O. A plain period (.) or asterisk (*) also
works when the middle dot is inconvenient to type.
Why does capitalization matter?
Chemical element symbols are case-sensitive. For example, Co is cobalt, but
CO is carbon monoxide. The calculator follows that convention and reports an
unknown-element error when a symbol is not in the mass table.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool molecular-weight-calculator "C8H10N4O2"New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/molecular-weight-calculator/?formula=C8H10N4O2&decimals=4Machine-readable descriptor: tool.json — title + parameters JSON Schema for agents.