Molar mass calculator

A molar mass is a sum, and the only hard part is reading the formula correctly: the subscript after a bracket multiplies everything inside it, a hydrate dot starts a whole new part with its own multiplier, and a two-letter symbol has to be told apart from two one-letter ones — CO is carbon monoxide, Co is cobalt, and they differ by more than fifteen grams per mole. This page parses the formula the way a chemist reads it, then shows the arithmetic element by element so you can see where the mass sits rather than trusting a single number.

How it is calculated

M = Σ (Ar(X) × count(X)) g/mol

The relative atomic mass of an element, multiplied by how many of its atoms the formula contains and summed over the formula, is the relative molecular mass — and a relative molecular mass in grams per mole is the molar mass. The atomic weights come from the IUPAC Commission on Isotopic Abundances and Atomic Weights, which is why the answers here differ in the last digit or two from tables printed before the 2021 report.

Source: NIST SP 811 (2008), Sec. 8.6, Amount of substance, concentration, molality, and the like — Sec. 8.6.4, molar mass M = m/n, and for a substance of definite composition M(B) = Mr(B) g/mol, so the relative molecular mass summed from the atomic weights is the molar mass in grams per mole

Questions people ask

Why does the page mention an interval for hydrogen, carbon or oxygen?

Because IUPAC no longer gives those elements a single atomic weight. Their isotopic composition genuinely varies between natural sources, so the standard atomic weight is published as a range — oxygen, for instance, as [15.99903, 15.99977]. Fourteen elements are like this. Where a formula contains one, this page uses the abridged five-figure value the Commission publishes alongside the interval, and names the elements involved rather than hiding the fact that the third decimal place is a convention rather than a measurement of your sample.

How do I type a hydrate?

Any of the usual marks works: CuSO4·5H2O with a middle dot, or CuSO4*5H2O and CuSO4.5H2O if the dot is awkward to type. The number in front of the second part multiplies that whole part, so the five waters contribute ten hydrogens and five oxygens. Copper sulfate pentahydrate comes out at 249.677 g/mol against 159.602 for the anhydrous salt, which is a difference worth catching before you weigh anything.

Which brackets can I use?

Round, square and curly, nested as deeply as you like: K4[Fe(CN)6] and [Cu(NH3)4]SO4 both read correctly. What the page will not accept is a mismatched pair — Ca(OH] gets rejected rather than quietly repaired, because a repaired formula is a guess about what you meant.

Why will it not calculate technetium or radium?

Because IUPAC publishes no standard atomic weight for them. Those elements have no isotopic composition characteristic of normal terrestrial material, so the table prints a dash where a value would go. Textbooks often show the mass number of a common isotope instead, which is a different quantity with a different meaning, so this page says it has nothing rather than substituting one.

Is this the same as molecular weight?

Near enough for practical purposes, with one distinction worth knowing. Relative molecular mass is a ratio and has no unit; molar mass is a mass per amount of substance and is measured in grams per mole. The two carry the same digits, which is why the terms get used interchangeably, and why an answer quoted without units is still usable.

Why does my textbook give a slightly different number?

Almost always because it uses an older table. Standard atomic weights get revised as measurements improve — gadolinium, lutetium and zirconium changed as recently as 2024 — and a printed table can be a decade behind. The difference lands in the third or fourth decimal place and matters for analytical work, not for a stoichiometry exercise.

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