Molecular weight is the sum of every atom's standard atomic weight in the formula. For glucose, C6H12O6: 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 g/mol. Enter any formula, including brackets like Ca(OH)2 or hydrates like CuSO4·5H2O, and the per-element breakdown appears below the total.
Weighing out a reagent, converting grams to moles for a stoichiometry problem or checking a label all start with the same number: the mass of one mole of the compound. This calculator reads a written formula, expands parentheses, square brackets and hydrate water, counts each element and multiplies by its IUPAC conventional atomic weight. Alongside the total you get the number of atoms in one formula unit, a table of each element's contribution and percentage by mass, a bar chart of those shares, and the mass of whatever amount in moles you enter. One limit: the weights are averages for natural isotopic abundance, so isotopically labelled compounds (deuterated solvents, 13C tracers) and exact monoisotopic masses for mass spectrometry need different values.
Use element symbols with correct capitals, digits for subscripts, ( ) or [ ] for groups and a dot for water of crystallisation.
Optional. The calculator multiplies it by the molecular weight to give a mass in grams.
The table and bar chart show how much of the total each element supplies; download the chart as PNG, SVG or CSV if needed.
Result Molecular weight (g/mol): 180.156
Atoms per formula unit: 24
Distinct elements: 3
Mass of the entered amount (g): 180.156
Six carbons, twelve hydrogens and six oxygens add up to 180.156 g/mol; oxygen alone supplies about 53% of that mass.
Result Molecular weight (g/mol): 18.015
Atoms per formula unit: 3
Distinct elements: 2
Mass of the entered amount (g): 36.03
Water weighs 18.015 g/mol, so two moles have a mass of 36.03 g.
Result Molecular weight (g/mol): 249.677
Atoms per formula unit: 21
Distinct elements: 4
Mass of the entered amount (g): 249.677
The five waters of crystallisation add 90.075 g/mol to the 159.602 g/mol of anhydrous copper sulfate.
M = Σ (nᵢ × Aᵢ)
MnᵢAᵢThe parser walks the formula left to right. A subscript after a bracket multiplies every count inside it, so Ca(OH)2 gives one Ca, two O and two H. A leading number after a hydrate dot multiplies that whole part, so ·5H2O adds ten H and five O. Each element count is multiplied by its atomic weight and the products are summed. Mass percentage is each product divided by the total.
Numerically yes. Molecular weight is strictly a dimensionless ratio to one twelfth of a carbon-12 atom, while molar mass carries units of g/mol. For glucose both read 180.156, so most labs use the terms interchangeably.
The IUPAC abridged and conventional standard atomic weights, such as 1.008 for hydrogen, 12.011 for carbon and 15.999 for oxygen. Elements with a published interval, like hydrogen and carbon, use the single conventional value IUPAC gives for general use.
Put a middle dot, a full stop or an asterisk between the salt and the water, followed by the number of waters: CuSO4·5H2O, CuSO4.5H2O and CuSO4*5H2O all give 249.677 g/mol.
Yes. Square and round brackets can be nested as deep as the formula requires, for example K4[Fe(CN)6]. Each closing bracket's subscript multiplies everything inside that pair.
Symbols are case-sensitive and must exist in the table, which covers hydrogen through uranium excluding elements without a standard weight such as technetium and promethium. A lowercase first letter, or an unknown pair such as Xq, is rejected.
Divide your mass in grams by the molecular weight shown. For 90 g of glucose, 90 ÷ 180.156 is 0.4996 mol. The amount field works the other way, turning moles into grams.
Using this tool in an assignment, article or report? Copy a citation, or a link for your website.
Tools Directory Editorial Team. (2026, October 23). Molecular Weight Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/chemistry/stoichiometry-and-reactions/molecular-weight-calculator/
Tools Directory Editorial Team. “Molecular Weight Calculator.” Tools Directory, 23 Oct. 2026, https://tools.milunir.com/chemistry/stoichiometry-and-reactions/molecular-weight-calculator/. Accessed 27 Oct. 2026.
Tools Directory Editorial Team. “Molecular Weight Calculator.” Tools Directory. Last modified October 23, 2026. Accessed October 27, 2026. https://tools.milunir.com/chemistry/stoichiometry-and-reactions/molecular-weight-calculator/.
<a href="https://tools.milunir.com/chemistry/stoichiometry-and-reactions/molecular-weight-calculator/">Molecular Weight Calculator – Tools Directory</a>