Molar mass calculator
Type a formula — brackets and hydrate dots work — and get the molar mass in g/mol with the mass share of every element. The atomic weights are IUPAC/CIAAW values, and the uncertainty is propagated rather than dropped.
Updated on October 7, 2026
Calculator
It understands plain formulas (H2SO4), bracketed groups (Ca(OH)2, K3[Fe(CN)6]) and hydrates written with a dot (CuSO4.5H2O or CuSO4·5H2O). It does not take charges, phases or trivial names.
Examples:
98.072 g/mol
± 0.020397 g/mol (standard uncertainty)
IUPAC publishes a range for this element because its isotopic composition varies in nature. The value shown is the conventional atomic weight, and the uncertainty is half the width of that range, not a measurement error.
| Symbol | Name | Atoms | Standard atomic weight | Share of mass |
|---|---|---|---|---|
| H | Hydrogen | 2 | 1.008 | 2.06 % |
| S | Sulfur | 1 | 32.060 | 32.69 % |
| O | Oxygen | 4 | 15.999 | 65.25 % |
How the number is built, and its uncertainty
The molar mass is the sum of each element’s standard atomic weight times how many atoms of it the formula contains. Because each atomic weight carries its own uncertainty, we combine them in quadrature — the square root of the sum of the squares — which is the standard way of propagating independent uncertainties.
Two honest caveats the calculator shows when they apply. If the formula contains an element whose atomic weight IUPAC publishes as a range (carbon, oxygen, sulfur, chlorine, lead and nine others), the result inherits natural isotopic variation, not just measurement error. If it contains an element with no stable nuclide, there is no standard atomic weight at all and the result is approximate, built from a mass number.
Source: Cálculo próprio a partir dos dados citados nesta página
How to read this page
How to read the numbers here
Every value on "Molar mass calculator" comes from the source named beside its section, never from an estimate of our own with no origin.
Where the data comes from
Each section of "Molar mass calculator" names its official source; the verification date sits at the top of the page.
What the number doesn’t tell you
H2O’s molar mass shifts slightly with where the sample comes from, because one of its elements has an atomic weight published as a range, not a single value.
More about “Molar mass calculator”
H2O compared with water
At 18.015 g/mol, one molecule (or formula unit) of H2O weighs 1.0 time as much as one water molecule. A single gram of H2O holds roughly 3.3428 × 10²² units — a number that only makes sense because the mole exists.
Worth repeating, because "Molar mass calculator" depends on it: one element in H2O has an atomic weight published as a range by the IUPAC, so the third decimal place of the molar mass shifts a little with where the sample came from.
Words on this page
Atomic weight
The average weight of an element’s atoms, weighted by how abundant each isotope is in nature, in atomic mass units. For 14 elements CIAAW publishes a range instead of a single value, because that abundance varies a little from sample to sample.
Standard definition by CIAAW — Commission on Isotopic Abundances and Atomic Weights · Atomic weight
Mole
The SI counting unit for atoms, molecules or other entities: one mole is exactly 6.02214076 × 10²³ of them, by the definition fixed in 2019. It is what ties a mass in grams to a number of particles.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Mole
Standard uncertainty
The margin attached to a measured value, usually written in parentheses on the last digits. A constant that is "exact by definition" has zero standard uncertainty; a measured one, like the gravitational constant G, always carries some.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Standard uncertainty
Isotope
Atoms of the same element (same number of protons) with a different number of neutrons, and therefore a different mass. Carbon-12 and carbon-14 are isotopes of carbon; only the second one is radioactive.
Atomic number
The number of protons in an atom’s nucleus, written Z. It is this number, not the mass, that defines which element an atom is: every atom with 8 protons is oxygen, whether it has 8 or 18 neutrons.
Standard definition by IUPAC · Atomic number