Molar mass calculator
Glucose, C6H12O6, is the worked example. Give the molar mass calculator any formula, including brackets, hydrates and charged ions, and the mass is summed from the IUPAC atomic weights, element by element.
Molar mass
- Molar mass 6 × 12.011 (C) + 12 × 1.0080 (H) + 6 × 15.999 (O) = 180.156 g/mol, from the IUPAC (CIAAW 2024) abridged standard atomic weights. Shown to 2 decimal places because the atomic weights are uncertain by about ±0.01 g/mol for this formula.
- 180.16 g/mol
- Formula as read 24 atoms of 3 elements. Brackets multiply everything inside them, and a hydrate's water is counted in full.
- C × 6, H × 12, O × 6
- Mass percent C 6 × 12.011 = 72.066 g/mol of the 180.16.
- 40.00%
- Mass percent H 12 × 1.0080 = 12.096 g/mol of the 180.16.
- 6.71%
- Mass percent O 6 × 15.999 = 95.994 g/mol of the 180.16. The largest share by mass, which is not necessarily the most atoms.
- 53.28%
- Mass of one mole One mole is 6.02214076 × 10^23 formula units (the exact SI value of the Avogadro constant), and weighs the molar mass in grams. So 1 g of C6H12O6 is 0.005551 mol.
- 180.16 g
| Element | Atoms | Atomic weight | Mass (g/mol) | Mass percent |
|---|---|---|---|---|
| C (carbon) | 6 | 12.011 | 72.066 | 40.00% |
| H (hydrogen) | 12 | 1.0080 | 12.096 | 6.71% |
| O (oxygen) | 6 | 15.999 | 95.994 | 53.28% |
| Total | 24 | 180.16 | 100.00% |
- Molar mass and molecular weight are the same number: molar mass is in g/mol, molecular weight (relative molecular mass) is the same figure without units.
- Isotopically labeled or enriched material (deuterated solvents, 13C compounds) has a different molar mass. These atomic weights are for normal terrestrial material.
How the Molarity Calculator works
Molar mass is the sum of the atomic weights of every atom in the formula, in grams per mole. For glucose, C6H12O6: six carbons at 12.011, twelve hydrogens at 1.0080 and six oxygens at 15.999 give 72.066 + 12.096 + 95.994 = 180.156, so 180.16 g/mol. One mole of glucose (6.02214076 × 10^23 molecules) weighs 180.16 g.
Molecular weight is the same number without the unit. Strictly it is the relative molecular mass, a ratio to one twelfth of the mass of a carbon-12 atom, which is why it has no units; molar mass is that ratio expressed in g/mol. For anything you will weigh on a balance they are interchangeable, so this works as a molecular mass calculator too, and "formula weight" is the same thing again for ionic compounds such as NaCl, which have no discrete molecules.
Reading the formula is most of the work. Each element symbol is one capital letter, optionally followed by one lowercase letter, and the number after it counts atoms of that element. Capitalization carries meaning: Co is cobalt, and CO is a carbon and an oxygen. Brackets multiply everything inside them, so Ca(OH)2 is one calcium, two oxygens and two hydrogens, and Al2(SO4)3 contains twelve oxygens. Square brackets work the same way for complex ions, as in K4[Fe(CN)6]. A hydrate's water is added in full: CuSO4·5H2O has nine oxygens, four from the sulfate and five from the water. Type the dot as ·, a period or an asterisk. A charge written at the end, as in Fe3+ or SO4^2-, is read and reported but does not change the mass in any meaningful way, because electrons weigh about 0.00055 g/mol each.
The mass percent column is each element's share of the total: for glucose, carbon is 72.066 / 180.156 = 40.00%, hydrogen 6.71% and oxygen 53.28%. This is percent composition, the starting point for empirical-formula problems and for checking an elemental analysis.
Where the numbers come from. The atomic weights are IUPAC's, from the 2024 abridged table published by its Commission on Isotopic Abundances and Atomic Weights. Fourteen elements, including hydrogen, carbon, nitrogen and oxygen, have an interval rather than a single standard atomic weight because their isotope ratios vary in nature; the abridged table gives the single value meant for calculation, and that is used here. This is also why some printed molar masses differ from these in the second decimal place: many textbooks still use older values such as sulfur 32.065, giving 249.69 for copper sulfate pentahydrate where the current weights give 249.68. Elements with no stable isotopes, such as technetium and plutonium, have no standard atomic weight at all, and the page says so rather than substitute a mass number.
Molarity Calculator FAQ
How do you calculate molar mass?
Multiply each element's atomic weight by the number of its atoms in the formula and add them up. Water, H2O: 2 × 1.0080 + 15.999 = 18.015 g/mol. Brackets multiply their contents, and a hydrate's water molecules are included.
Is molar mass the same as molecular weight?
Numerically, yes. Molecular weight (relative molecular mass) is a unitless ratio; molar mass is the same number in grams per mole. Formula weight is the same calculation for ionic compounds, which do not form discrete molecules.
Why does my textbook give a slightly different molar mass?
Atomic weights are revised by IUPAC every few years, and many references still use older values. This calculator uses the 2024 abridged table. Differences usually appear in the second decimal place, which is also about the precision the atomic weights themselves support.
How do I find percent composition?
Divide the mass contributed by each element by the total molar mass. In glucose, six carbons contribute 72.066 of 180.156 g/mol, so carbon is 40.00% by mass. The breakdown table above does this for every element.