Molecular Weight Calculator
Any formula, straight to g/mol.
Molecular Weight Calculator
Read any molecular formula into its molar mass, atom count, and percentage composition by mass.
Brackets and hydrates are supported: Ca(OH)2, CuSO4·5H2O. Capitalisation matters — Co is cobalt, CO is carbon monoxide.
Common formulas
Masses use IUPAC 2021 standard atomic weights. They are conventional values for stoichiometry, not isotope-specific masses, so a high-precision instrument may read slightly differently.
180.156g/mol
C6H12O6 · 24 atoms
| Element | Atoms | Mass % |
|---|---|---|
| C | 6 | 40% |
| H | 12 | 6.71% |
| O | 6 | 53.28% |
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What This Calculator Does
The Molecular Weight Calculator reads a chemical formula and returns its molar mass, the number of atoms in one formula unit, and each element’s share of the total mass. It handles subscripts, nested brackets, and hydrates such as copper sulfate pentahydrate.
How It Works
The formula is parsed into element counts, including the multiplier that applies after a bracket and the coefficient after a hydrate dot. Each count is multiplied by the IUPAC standard atomic weight for that element and summed. Percentage composition is each element’s contribution divided by the total.
Example
Glucose, C6H12O6, contains 6 carbon, 12 hydrogen, and 6 oxygen atoms. Its molar mass is 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.16 g/mol, of which carbon is 40.0% and oxygen 53.3%.
Formula
M = Σ (atom count × standard atomic weight)Element % = (element mass / molar mass) × 100CuSO₄·5H₂O = CuSO₄ + 5 × H₂OA number after the dot multiplies the whole following formula unit, not just its first element.
Reading the Results
Molar mass
Grams per mole of the formula unit as written. For an ionic compound this is formula mass, because there are no discrete molecules.
Mass percent
The fraction of total mass contributed by each element. It is the same ratio used in empirical formula and yield calculations.
Sources and References
IUPAC — Standard Atomic Weights (2021)
Abridged atomic weights used for each element
IUPAC publishes the conventional atomic weights that stoichiometry uses.
NIST Chemistry WebBook (SRD 69)
Evaluated thermochemical and mass data
Useful for checking a computed molar mass against an independent reference value.
PubChem — Compound records
Molecular formula and molecular weight
A cross-check for common compounds, including hydrate forms.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
What is the difference between molecular weight and molar mass?
Molecular weight is the mass of one molecule in atomic mass units; molar mass is the mass of one mole in grams per mole. The numbers are numerically identical, which is why the terms are often used interchangeably.
Why does Co give a different answer from CO?
Chemical symbols are case-sensitive. Co is cobalt, one atom with an atomic weight near 58.93. CO is one carbon and one oxygen, totalling about 28.01. The calculator rejects unknown symbols rather than guessing.
How are hydrates handled?
A dot separates formula units, and a leading number multiplies the unit that follows. CuSO4·5H2O is one copper sulfate unit plus five water units, giving 249.68 g/mol rather than the anhydrous 159.60.
Does this account for isotopes?
No. It uses standard atomic weights averaged over natural abundance. For isotope-labelled work, or for mass spectrometry, use the monoisotopic mass of the specific isotopologue instead.