Molecular Weight Calculator
Calculate the molecular weight of a substance from its chemical formula.
Tip: For hydrated salts, include the water: CuSO₄·5H₂O = 249.69 g/mol (not just 159.61 g/mol without water).
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About this digital tool
The molecular weight calculator finds the molecular (molar) mass of a chemical compound from its formula. Enter the formula, e.g. H2SO4, C6H12O6, NaCl or Ca(OH)2, and get the mass in g/mol as well as the total number of atoms. Brackets and hydrates (such as CuSO4.5H2O) are supported.
Molecular weight is the sum of the atomic masses of all the atoms in the formula. For water H2O: 2 × 1.008 + 15.999 = 18.015 g/mol. For CO2: 12.011 + 2 × 15.999 = 44.009 g/mol. For NaCl: 22.990 + 35.45 = 58.44 g/mol. It is used in chemistry to convert grams to moles, for preparing solutions, for stoichiometry and in school and university assignments.
How to use
- Enter the chemical formula (e.g. H2SO4, NaCl, C6H12O6)
- The calculator analyses the formula automatically
- See the molar mass (g/mol)
- See the percentage composition of each element
- See the total number of atoms
Frequently asked questions
How is molecular weight calculated?
Add the atomic masses × the number of atoms for each element. E.g., C₆H₁₂O₆ (glucose) = 6×12.011 + 12×1.008 + 6×15.999 = 180.156 g/mol.
Where do I find atomic masses?
From the periodic table. Our calculator has them built in: H=1.008, C=12.011, N=14.007, O=15.999, Na=22.990, Cl=35.453, S=32.065, etc.
What is the difference between molecular weight and molar mass?
They are numerically the same but with different units. Molar mass: g/mol (of one mole). Molecular weight: Da or amu (of one molecule). E.g., H₂O: molar mass = 18.015 g/mol, molecular weight = 18.015 amu.
How are brackets treated in a formula, as in Ca(OH)2?
Atoms inside the brackets are multiplied by the subscript after them. Ca(OH)2 = 40.078 + 2 × (15.999 + 1.008) = 74.092 g/mol.