Molar mass calculator
Type a formula and get its molar mass, the multiplication behind it, and something almost no other calculator will tell you: how many of those decimal places are real. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
Li2CO3 is 73.9, not 73.8880
Every molar mass calculator on the internet will tell you that lithium carbonate is 73.8880 g/mol. Six figures. Four decimal places. It looks like precision.
Lithium's atomic weight is 6.94 ± 0.06. That is an uncertainty of 0.86%, and there are two lithium atoms in the formula, so the molar mass carries ± 0.12 before anything else is counted. The second decimal place is already gone. The honest answer is 73.9 g/mol, and this page prints that.
Lithium is not a rounding problem. Its isotopic composition varies genuinely and widely between sources — commercially available lithium has often been through isotopic separation for other purposes, and what is left behind is depleted. CIAAW mark it m for exactly that reason.
| Element | Atomic weight | Uncertainty |
|---|---|---|
| lithium | 6.94 | 8,646 ppm |
| lead | 207.2 | 5,309 ppm |
| argon | 39.95 | 4,005 ppm |
| boron | 10.81 | 1,850 ppm |
| the median element | 62 ppm |
Lead is the one that catches people out in practice, because it turns up in real analysis. PbO is 223 g/mol. Not 223.2, and certainly not 223.1990 — the uncertainty is ± 1.1, so there is no decimal place worth printing at all.
CO is not Co, and nothing will tell you
Element symbols are case-sensitive, and that is not a typographical nicety. CO is carbon monoxide, 28.010 g/mol. Co is cobalt, 58.933 g/mol. Both are real. Both parse. Nothing in any calculator can tell you which one you meant, because both are correct answers to different questions.
Sixteen of the 104 two-letter element symbols are built from two letters that are themselves both element symbols:
Si · Sc · Co · Ni · Cu · Nb · In · Sn · Sb · Cs · Ho · Yb · Hf · Os · Pb · Bi
Twenty-six symbols have that shape, but ten of them involve an element with no standard atomic weight, so one of the two readings refuses to answer and the mistake is visible. These sixteen are the ones where both readings give you a number.
And the dangerous one is not the dramatic one. Ho against H+O is 164.93 against 17.007 — a factor of ten, nobody misses it. Sc against S+C is 44.956 against 44.071. That is a 2.0% difference, which is small enough to look right, and a number that looks right is a number nobody checks.
This page warns you when your formula contains one of the sixteen, and only then.
What it will take
Brackets of all three kinds and nested as deep as you like:
K4[Fe(CN)6], Al2(SO4)3,
{Cu(NH3)4}SO4. Hydrates with any of the dots people
actually type: CuSO4·5H2O,
CuSO4*5H2O, CuSO4.5H2O. A leading
coefficient, so 2H2O is four hydrogens. Spaces
anywhere.
It will refuse anything it cannot read, and say why, rather than quietly skipping the part it did not understand. That matters more than it sounds: a parser that ignores what it cannot parse returns a plausible number for a formula you did not type.
Where these numbers come from
The atomic weights on this page are the abridged standard atomic weights 2024, © IUPAC / CIAAW — the Commission on Isotopic Abundances and Atomic Weights of the International Union of Pure and Applied Chemistry, published at ciaaw.org and reproduced under their terms. CIAAW has published the critical evaluation of atomic weights since 1902.
The abridged table is the one used here rather than the full one, and that is a decision worth stating. Fourteen elements have no single atomic weight in the full table at all — they get an interval instead — and taking the middle of an interval would be this site inventing a number nobody published. CIAAW publish the abridged table with a single value and an uncertainty for exactly this purpose, and that is what you see.