About the Molar Mass Calculator
This molar mass calculator adds up the standard atomic weights of every atom in a chemical formula to give the molar mass (molecular weight) in grams per mole. It also shows how many atoms of each element the formula contains and what percentage of the total mass each element contributes.
It is the first step in almost every chemistry calculation — converting grams to moles, preparing a solution of a given molarity, or working out theoretical yield — so it is useful for students, teachers and lab technicians alike. The formula parser understands parentheses and brackets, nested groups such as K4[Fe(CN)6], and hydrates like CuSO4·5H2O or MgSO4*7H2O.
Atomic weights are IUPAC conventional values (for example H = 1.008, C = 12.011, O = 15.999). For radioactive elements without a standard weight, the mass number of the longest-lived isotope is used. Element symbols are case-sensitive: write Co for cobalt and CO for carbon monoxide.
With the default inputs, the molar mass is 98.072 g/mol. Change any value above to recalculate instantly.
How to use the molar mass calculator
- 1Type the chemical formula using correct capitalisation (NaCl, not NACL).
- 2Use brackets for groups, e.g. Al2(SO4)3.
- 3Write hydrates with a dot or asterisk, e.g. CuSO4·5H2O.
- 4Read the molar mass in g/mol.
- 5Check the table and chart for each element’s contribution.
Formula and method
The molar mass of a compound is the sum, over every element in the formula, of the number of atoms of that element multiplied by its standard atomic weight. A subscript after a bracket multiplies everything inside the bracket, and the number in front of a hydrate’s water (the 5 in ·5H2O) multiplies that part of the formula.
Numerically, the molar mass in g/mol equals the mass of one molecule in daltons (unified atomic mass units). Each element’s mass percentage is its subtotal divided by the total molar mass — this is the percent composition used in empirical-formula problems.
- M
- Molar mass of the compound (g/mol)
- nᵢ
- Number of atoms of element i in the formula
- Aᵢ
- Standard atomic weight of element i
Worked examples
Sulfuric acid, H2SO4
2 × 1.008 (H) + 32.06 (S) + 4 × 15.999 (O) = 98.072 g/mol. Oxygen makes up about 65% of the mass.
Glucose, C6H12O6
6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 g/mol. Carbon contributes 72.066 g/mol, or 40.0% of the mass.
Copper(II) sulfate pentahydrate, CuSO4·5H2O
CuSO4 is 63.546 + 32.06 + 4 × 15.999 = 159.602 g/mol, and 5 H2O add 5 × 18.015 = 90.075 g/mol, giving 249.68 g/mol.
Frequently asked questions
How do you calculate molar mass?+
Multiply the atomic weight of each element by the number of its atoms in the formula and add the results. For water, 2 × 1.008 + 15.999 = 18.015 g/mol.
Is molar mass the same as molecular weight?+
They are numerically the same. Molar mass has units of g/mol, while molecular weight (relative molecular mass) is dimensionless or given in daltons for a single molecule.
How do I enter parentheses and hydrates?+
Type them as written: Mg(OH)2, (NH4)2SO4 or Al2(SO4)3. For hydrates put a dot, middle dot or asterisk before the water, such as Na2CO3·10H2O or Na2CO3*10H2O.
Why does my answer differ slightly from my textbook?+
Textbooks often round atomic weights (e.g. Cl = 35.5, H = 1.01). This calculator uses IUPAC conventional values such as Cl = 35.45 and H = 1.008, so results can differ in the second decimal place.
Why is CO different from Co?+
Symbols are case-sensitive. Co is the element cobalt (58.933 g/mol), while CO is one carbon and one oxygen atom — carbon monoxide (28.010 g/mol).