About this site
Counting Atoms is part of a public library of interactive explanations. It is free, has no advertising and needs no sign-up. It explains the mole, molar mass, balancing equations and stoichiometry, and offers three calculators built to be exact.
Where the numbers come from§
Atomic weights are the Abridged Standard Atomic Weights 2024 of the Commission on Isotopic Abundances and Atomic Weights (CIAAW) of IUPAC. They were copied by a program from the Commission's page on 10 November 2026, and the site's tests compare every one of the 118 entries with a frozen copy of that page, and check that each abridged value and its uncertainty covers the full standard atomic weight, including the fourteen given as intervals. Isotope masses and compositions for the isotope figure are from NIST's database of atomic weights and isotopic compositions. The Avogadro constant is the exact value fixed by the 2018 revision of the SI, in force since 20 May 2019.
How the calculations are checked§
All arithmetic is exact: numbers are held as fractions of whole numbers of any size, so no rounding happens until a result is displayed. Formulas are read by a strict grammar; anything outside it is refused with a reason, and a mis-capitalised formula is answered with its valid readings, never silently corrected. Equations are balanced by solving the equations of atom (and charge) conservation exactly; when they have no solution, or more than one independent solution, the balancer says so.
The engine's tests include textbook worked examples from OpenStax Chemistry 2e, the molar masses of the most-searched compounds, a comparison with PubChem's molecular weights for 35 compounds (every value agrees within the stated uncertainty of the atomic weights), more than 45 equations with known coefficients including hard redox cases, and invariants checked for every balanced equation: atoms, charge and mass conserved, coefficients positive and with no common factor, and the answer unchanged when the substances are listed in another order. An independent test suite, written from the published equations without reading this site's code, and an independent review of every claim against its source, are part of the site's checks. What each figure simplifies is stated next to it and collected on the sources page.
Found an error?§
Please open an issue on the source repository. Corrections are welcome and are credited. A reported error is checked by recomputing it independently, and the result, whichever way it goes, becomes a permanent test.
Reuse§
Text and figures are licensed CC BY 4.0; code is MIT. Teachers may use and adapt any of it, with attribution. Every figure has a Share button that copies a link to its exact state.