Molar Mass and Percent Composition Calculator

Exact molar mass, with a table for each element and the source of the weights.

Molar mass is the mass of one mole (6.02214076 x 10^23 entities) of a substance, in g/mol, and is numerically equal to the molecular mass in atomic mass units (u). It is the sum of the atomic weights of every atom in the formula: in H2SO4, 2 x 1.008 + 32.06 + 4 x 15.999 = 98.072 g/mol. The weights come from the CIAAW (IUPAC) Standard Atomic Weights 2024 table. For the 14 elements whose weight varies in nature (H, Li, B, C, N, O, Mg, Si, S, Cl, Ar, Br, Tl and Pb), the CIAAW publishes an interval and this tool uses the conventional value (for example H = 1.008, C = 12.011, O = 15.999, Cl = 35.45), marking it with an asterisk. Elements with no stable isotope use the mass number of the most stable isotope. The arithmetic uses exact fractions, and rounding happens only at the end, to the number of decimals you choose. The parser understands nested parentheses and square brackets, hydrates (CuSO4.5H2O adds 5 waters: 249.677 g/mol, of which 36.08% is water), charges, deuterium (D) and isotopes such as [13C]. The mass percent of each element is (subtotal / molar mass) x 100. The Rounded mode uses exam values (H = 1, O = 16, Cl = 35.5), with which water is 18 g/mol instead of 18.015.

Type the formula and read the table by element.

1. Type the formula (Ca3(PO4)2, CuSO4.5H2O, NH4+) or tap an example. If there is an error, the tool marks the spot and suggests the fix.
2. See the molar mass and the table with atoms, atomic mass, subtotal and mass percent, and copy the value, the text or the CSV.
3. Use the other tabs to convert grams, moles and particles, find the empirical and molecular formula from a composition, or calculate several formulas at once.

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Sources and references for this tool

These references help contextualize formulas, standards, APIs and limitations used on this page. They do not replace professional validation when a result has legal, financial, medical or operational impact.

• Standard Atomic Weights 2024CIAAW / IUPAC, Standard atomic weights table used in every calculation.
• Abridged Standard Atomic Weights 2024CIAAW / IUPAC, Conventional values for elements with interval weights (H, C, O, Cl...).
• Radioactive elements (NUBASE2020)CIAAW / IUPAC, Mass number of the most stable isotope of radioactive elements.
• Avogadro constantNIST CODATA, Avogadro constant, 6.02214076 x 10^23 mol^-1, exact since 2019.

Frequently asked questions

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