Formula weight is the sum of the atomic weights of every atom in one formula unit. For water, H2O, that is 2 × 1.008 + 15.999 = 18.015 g/mol. Type any formula, including brackets such as Ca(OH)2 or hydrates such as CuSO4·5H2O, and the calculator totals each element and shows its mass share.
Students balancing stoichiometry problems, lab staff weighing out reagents and anyone converting grams to moles needs the formula weight first. This calculator reads a formula as written, expands brackets and hydrate dots, and multiplies each atom count by its IUPAC standard atomic weight. You get the total in g/mol, the number of atoms per formula unit, a table of each element's contribution and mass percentage, and the sum written out term by term so you can check it against your own working. Element symbols are case-sensitive, exactly as on the periodic table, so Co is cobalt while CO is carbon monoxide. One limit: it uses abridged standard weights for natural isotopic mixtures, so isotopically labelled compounds or exact monoisotopic masses for mass spectrometry need different values.
Enter symbols with their counts, for example Al2(SO4)3. Capitalise the first letter of each symbol only.
Join waters of crystallisation with a dot or middle dot, as in MgSO4.7H2O; the leading number multiplies that part.
The table lists atoms, atomic weight, mass and mass percent per element, and the working line shows the full sum.
Result Formula weight (g/mol): 18.015
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 about 89% of the mass.
Result Formula weight (g/mol): 180.156
Atoms per formula unit: 24
Distinct elements: 3
Largest mass share: O (53.28% by mass)
The textbook sugar: 24 atoms across three elements adding to 180.156 g/mol.
Result Formula weight (g/mol): 249.677
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 hydrate's weight.
FW = Σ (nᵢ × Aᵢ)
FWnᵢAᵢThe parser walks the formula left to right. A bracketed group is counted once, then multiplied by the number after its closing bracket; nested groups multiply in turn. A hydrate dot splits the formula into parts, and a number at the start of a part multiplies every atom in it. Counts for the same element are merged, multiplied by its atomic weight, and summed. Mass percent is each element's mass divided by the total. Elements without a stable isotope use the mass number of their longest-lived isotope, e.g. 98 for technetium.
Numerically yes. Formula weight is the mass of one formula unit in atomic mass units, and molar mass is the mass of one mole in grams; both come out as the same number, so 18.015 u for water corresponds to 18.015 g/mol.
Ionic solids such as NaCl do not form discrete molecules, so chemists speak of a formula unit and its formula weight. For covalent molecules like glucose the two terms give the same value.
IUPAC standard atomic weights, using the abridged or conventional single values where IUPAC gives an interval, such as 1.008 for hydrogen, 12.011 for carbon and 32.06 for sulfur. Textbooks with older tables may differ in the second decimal place.
Put a dot or a middle dot between the salt and the water, with the water count in front: CuSO4·5H2O or Na2CO3.10H2O. Each part is weighed and added; copper sulfate pentahydrate comes to 249.677 g/mol.
Yes. Round and square brackets both work and can be nested, as in K4[Fe(CN)6]. Each group is multiplied by the number after its closing bracket, and the brackets must pair correctly.
It is the share of the total weight carried by each element, which is the percent composition used in empirical-formula problems. In water, oxygen supplies about 88.8% of the mass even though it is one atom of three.
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Tools Directory Editorial Team. (2026, October 11). Formula Weight Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/chemistry/chemistry-reference/formula-weight-calculator/
Tools Directory Editorial Team. “Formula Weight Calculator.” Tools Directory, 11 Oct. 2026, https://tools.milunir.com/chemistry/chemistry-reference/formula-weight-calculator/. Accessed 27 Oct. 2026.
Tools Directory Editorial Team. “Formula Weight Calculator.” Tools Directory. Last modified October 11, 2026. Accessed October 27, 2026. https://tools.milunir.com/chemistry/chemistry-reference/formula-weight-calculator/.
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