Molar mass calculator (molecular weight)
Calculate the molar mass of a chemical formula, like H2O, C6H12O6 or Ca(OH)2, in g/mol.
Write the formula with correct case (Co ≠ CO) and parentheses when needed, e.g., Ca(OH)2, Al2(SO4)3.
How the calculation works
Molar mass is the sum of each element's atomic mass times how many atoms are in the formula. E.g., water (H₂O) = 2 × 1.008 + 15.999 = 18.015 g/mol. Parentheses multiply the whole group.
Examples
- CO₂: 12.011 + 2 × 15.999 = 44.01 g/mol.
- Ca(OH)₂: 40.078 + 2 × (15.999 + 1.008) = 74.09 g/mol.
Frequently asked questions
Is molar mass the same as molecular weight?
In practice the number is the same. Molar mass is in g/mol; molecular weight is dimensionless (in atomic mass units).
How do parentheses work?
The subscript after a parenthesis multiplies the whole group. In Ca(OH)₂, the 2 multiplies both O and H.
What is molar mass for?
It bridges grams and moles (moles = mass ÷ molar mass), key to stoichiometry and preparing solutions.
What is Avogadro's number?
It is 6.022 × 10²³, the number of entities in one mole. It bridges the atomic scale and the laboratory balance.
Why are atomic masses not whole numbers?
Because the periodic table gives a weighted average of natural isotopes. Chlorine reads 35.45 by mixing Cl-35 (~75%) and Cl-37 (~25%).
How do parentheses work in a formula?
The subscript after a parenthesis multiplies the whole group: Ca(OH)₂ has 1 calcium, 2 oxygens and 2 hydrogens.
Do I count water of hydration?
Yes. CuSO₄·5H₂O has a molar mass of 249.68 g/mol against 159.61 for the anhydrous salt — over 35% difference.
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See all →Updated on June 30, 2026 · by Rafael Rossi · Methodology & sources