Molecular mass is the sum of the atomic weights of every atom in the formula. For water, H2O, that is 2 × 1.008 + 15.999 = 18.015 g/mol. Enter any formula, including brackets and hydrates, and the calculator multiplies each element's count by its IUPAC standard atomic weight and adds the results.
Weighing out a reagent, converting grams to moles or checking a stoichiometry answer all start from the molar mass of a compound. This calculator reads a formula as you type it, expands brackets such as (PO4)2 and hydrate parts such as ·5H2O, and adds up the contributions element by element. Next to the total you get the atom count, the mass in kilograms per mole, and a table with each element's atomic weight, its share of the mass, and the working line by line. Atomic weights are the IUPAC conventional values to four or five significant figures, so the last decimal can differ slightly from older textbook tables. The parser does not read charges, isotopes like D or 13C, or condensed organic names, so write ions and labelled compounds out as plain element formulas.
Use element symbols with the right capitalisation and digits for counts, e.g. Al2(SO4)3.
Separate waters of crystallisation with · or a full stop, with the multiplier in front: MgSO4·7H2O.
The molar mass appears in g/mol, with each element's atoms, mass and percentage listed below it.
Result Molar mass (g/mol): 18.015
Milligrams per millimole: 18.015
Kilograms per mole: 0.018015
Atoms per formula unit: 3
Distinct elements: 2
Largest mass share: O (88.81% by mass)
Two hydrogens at 1.008 and one oxygen at 15.999 give 18.015 g/mol; oxygen carries almost 89% of the mass.
Result Molar mass (g/mol): 180.156
Milligrams per millimole: 180.156
Kilograms per mole: 0.180156
Atoms per formula unit: 24
Distinct elements: 3
Largest mass share: O (53.28% by mass)
The figure used to make a 1 M glucose solution: 180.156 g in one litre.
Result Molar mass (g/mol): 249.677
Milligrams per millimole: 249.677
Kilograms per mole: 0.249677
Atoms per formula unit: 21
Distinct elements: 4
Largest mass share: O (57.67% by mass)
The five waters of crystallisation add 90.075 g/mol, more than a third of the total.
M = Σ (nᵢ × Aᵣ,ᵢ)
MnᵢAᵣ,ᵢThe formula is parsed left to right. A digit after a symbol counts that atom; a digit after a closing bracket multiplies everything inside it; a leading number after a hydrate dot multiplies that whole part. Counts are merged per element, multiplied by the atomic weight, and summed. Because relative molecular mass is dimensionless and molar mass is in g/mol, the same number serves both; dividing by 1000 gives kg/mol.
Numerically yes. Relative molecular mass is a dimensionless ratio to one twelfth of a carbon-12 atom, while molar mass carries units of g/mol. Water is 18.015 on both scales.
The IUPAC Commission on Isotopic Abundances and Atomic Weights conventional values, such as 1.008 for hydrogen, 12.011 for carbon and 35.45 for chlorine. Elements with natural isotope variation have intervals; the single conventional value is used here.
Round and square brackets can be nested, for example K4[Fe(CN)6]. The count after each closing bracket multiplies every atom inside it, including inner groups, giving 4 K, 1 Fe, 6 C and 6 N.
Older tables used weights such as 1.00794 for hydrogen or 35.453 for chlorine. The difference usually appears only in the second or third decimal place and rarely matters for lab work.
Yes, ionic compounds are entered as their formula units, such as NaCl or Ca3(PO4)2. The mass of the electrons gained or lost by an ion is negligible, so charges are simply left out.
Each element's contribution divided by the total, times 100. For glucose, oxygen supplies 95.994 of 180.156 g/mol, about 53.28% of the mass, which is the percent composition used in elemental analysis.
Using this tool in an assignment, article or report? Copy a citation, or a link for your website.
Tools Directory Editorial Team. (2026, October 11). Molecular Mass Calculator. Tools Directory. Retrieved October 26, 2026, from https://tools.milunir.com/chemistry/chemistry-reference/molecular-mass-calculator/
Tools Directory Editorial Team. “Molecular Mass Calculator.” Tools Directory, 11 Oct. 2026, https://tools.milunir.com/chemistry/chemistry-reference/molecular-mass-calculator/. Accessed 26 Oct. 2026.
Tools Directory Editorial Team. “Molecular Mass Calculator.” Tools Directory. Last modified October 11, 2026. Accessed October 26, 2026. https://tools.milunir.com/chemistry/chemistry-reference/molecular-mass-calculator/.
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