Definition and units
The molar mass (symbol M) of a substance is the mass of exactly one mole of that substance. One mole contains 6.022 140 76 × 10²³ entities (atoms, molecules, formula units) — Avogadro's constant.
M = m / n
The molar mass of an element is numerically equal to its relative atomic mass — the average mass of its atoms weighted by isotope abundance, expressed in unified atomic mass units (u). For example, carbon has a relative atomic mass of 12.011 u, so its molar mass is 12.011 g/mol.
For the concept of isotopes and why atomic masses are non-integer averages, see our isotope glossary entry. For Avogadro's constant and the mole itself, see our moles guide.
How to calculate molar mass
For a compound: identify every element in the formula, multiply each element's atomic mass by its subscript, and sum the results.
Worked example — CaCO₃ (calcium carbonate):
- Ca: 1 × 40.078 = 40.078
- C: 1 × 12.011 = 12.011
- O: 3 × 15.999 = 47.997
- Total M = 100.086 g/mol
Our molar mass calculator does this for any formula in real time, using IUPAC 2021 atomic weights. For the periodic table of atomic masses, see our periodic table guide.
Common molar mass values
For the balancing equations that use these molar masses in stoichiometry, see our balancing equations guide. For concentration calculations that require molar mass conversions, see molarity and dilutions.
Molar mass in stoichiometry
Stoichiometry — calculating quantities in chemical reactions — requires working in moles because balanced equations state mole ratios. The molar mass enables the conversion:
Example: How much CO₂ (M = 44.009 g/mol) is produced from burning 10 g of methane CH₄ (M = 16.043 g/mol)?
CH₄ + 2O₂ → CO₂ + 2H₂O. Mole ratio CH₄ : CO₂ = 1 : 1.
- n(CH₄) = 10 / 16.043 = 0.6233 mol
- n(CO₂) = 0.6233 × 1 = 0.6233 mol
- m(CO₂) = 0.6233 × 44.009 = 27.4 g