Mole calculator
Give a formula and any one of mass, moles or number of particles, and this gives you the other two — with the molar mass worked out in front of you rather than asserted. 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.
The mole is a defined number now
Until 20 May 2019 the mole was defined as the number of atoms in 12 grams of carbon-12, and the Avogadro constant was something you went and measured. It had an uncertainty.
Since then the definition runs the other way round. One mole is 6.02214076×1023 entities, exactly, because that is what the SI now says a mole is. The number has no uncertainty because it is not a measurement.
What picked up the uncertainty instead is the mass of carbon-12 in a mole — which is now 11.9999999958 g or thereabouts rather than exactly 12, by an amount far too small to matter to anything on this page. The trade was deliberate: a defined constant is more useful than a defined lump of carbon.
The uncertainty does not disappear
The Avogadro constant is exact, but the molar mass you multiply by
is not, and that is where the doubt in any real answer lives. This
page shows it rather than rounding it out of sight. For sodium
chloride it is small; try Li2CO3 or PbO
and it stops being small.
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.