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Science & Physics

Molar Mass Calculator

Free molar mass calculator: parse any chemical formula (H2O, CO2, hydrates like CuSO4·5H2O, parentheses like Ca(OH)2) to get grams per mole plus a per-element breakdown and percentage composition, all in your browser.

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Type a formula — subscripts, (groups), and hydrate dots like CuSO4·5H2O all work.

Deep dive guides

A LITTLE CONTEXT · 4 MIN READReading Chemical Formulas: How Molar Mass Is Computed
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How molar mass calculator works

A molar mass is the mass of one mole of a substance — the atomic masses of every atom in a formula unit, added up in grams per mole (g/mol). Water is H2O: two hydrogens at 1.008 g/mol plus one oxygen at 15.999 g/mol gives 18.015 g/mol. The number is the bridge between the microscopic recipe of a compound and the macroscopic grams you weigh on a balance.

Type a formula and the tool parses it the way a chemist reads it: element symbols with counts (H2O, CO2), parentheses with group multipliers (Ca(OH)2, Fe2(SO4)3, nested groups like K4[Fe(CN)6]), leading multipliers, and hydrate dots — CuSO4·5H2O reads as copper sulfate plus five waters. Case matters, exactly as in chemistry: Co is cobalt (58.933 g/mol) while CO is carbon monoxide (28.01 g/mol).

The result shows the total molar mass to six significant figures, the total atom count per formula unit, and a breakdown table: each element, how many atoms it contributes, the mass it adds, and its mass percentage of the whole. In water, hydrogen is 2.016 g of 18.015 — about 11.2% by mass — and oxygen is 88.8%, the kind of number gravimetric analysis and stoichiometry problems are built on.

Frequently asked questions

What is the molar mass of CuSO4·5H2O?
Copper sulfate pentahydrate: Cu (63.546) + S (32.06) + 4 O (63.996) + 5 × H2O (90.075) = 249.677 g/mol. The five waters of crystallisation add real mass — anhydrous CuSO4 is only 159.602 g/mol — which is why hydrates must be counted when weighing reagents.
Why do Co and CO give different answers?
Chemical formulas are case-sensitive. "Co" (capital C, lowercase o) is the element cobalt; "CO" (both capitals) is one carbon and one oxygen — carbon monoxide. The parser reads symbols the same way: an uppercase letter optionally followed by one lowercase letter, then an optional count.
How do I enter hydrates and parentheses?
Separate hydrate parts with a dot — type CuSO4·5H2O, CuSO4*5H2O or CuSO4.5H2O; all parse the same. Groups use parentheses with an optional multiplier after the closing parenthesis, as in Ca(OH)2 or Fe2(SO4)3, and brackets work too: K4[Fe(CN)6]. Nesting is supported.
Molar mass vs molecular mass — which is this?
They are numerically identical but unit-ed differently: molecular mass is the mass of one molecule in daltons (u), while molar mass is the mass of one mole in g/mol. Because a mole is defined so the numbers match, water is 18.015 u per molecule and 18.015 g per mole. Ionic compounds like NaCl have no discrete molecules, so "formula mass" or "molar mass" is the preferred term.
Why does my answer differ slightly from a textbook?
Atomic masses depend slightly on the source and date: values are averaged over natural isotopic abundance and periodically refined. This tool uses standard IUPAC-style weights such as H = 1.008 and O = 15.999, so small differences in the last decimal (98.07 vs 98.08 for H2SO4) are normal and rarely matter in practice.

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 Reference

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