Molar Mass Calculator

Enter a chemical formula and get its molar mass, computed automatically from the atomic weight of every element it contains. Handles simple formulas like H2O, formulas with parentheses like Fe2(SO4)3, and hydrate notation like CuSO4·5H2O.

Atomic weights of common elements

Atomic weights for elements that commonly appear in chemical formulas (the calculator itself supports all 118 elements internally).

Symbol Element Atomic weight
H Hydrogen 1.008
He Helium 4.0026
C Carbon 12.011
N Nitrogen 14.007
O Oxygen 15.999
F Fluorine 18.998
Ne Neon 20.18
Na Sodium 22.99
Mg Magnesium 24.305
Al Aluminum 26.982
Si Silicon 28.085
P Phosphorus 30.974
S Sulfur 32.06
Cl Chlorine 35.45
Ar Argon 39.948
K Potassium 39.098
Ca Calcium 40.078
Fe Iron 55.845
Cu Copper 63.546
Zn Zinc 65.38
Br Bromine 79.904
Ag Silver 107.87
I Iodine 126.9
Ba Barium 137.33
Au Gold 196.97
Hg Mercury 200.59
Pb Lead 207.2

Tips

  • Element symbols are one uppercase letter plus optional lowercase letters — note that capitalization changes meaning, e.g. "Co" (cobalt) versus "CO" (carbon monoxide).
  • Parentheses can be nested as many times as needed (e.g. in Ca3(PO4)2, the number right after the parenthesis multiplies everything inside it).
  • For a hydrate, write the base compound followed by "·" or "*", a coefficient, and the water formula (e.g. MgSO4·7H2O).
  • If you omit a number after an element or group, it defaults to 1 (e.g. in NaCl, both Na and Cl have a count of 1).

FAQ

Numerically they're the same, but strictly speaking "molecular weight" applies to molecular substances (like H2O), while "molar mass" is the broader term that also covers ionic compounds (like NaCl). This tool works for both, which is why it labels the result "molar mass (molecular weight)".

They're the standard atomic weights published by IUPAC (the International Union of Pure and Applied Chemistry). For elements whose natural isotopic composition varies, this tool uses the widely adopted conventional value.

Atomic weights are calculated to about 3–4 significant figures after the decimal point, which is precise enough for most practical and educational purposes. For extremely high-precision analysis, check each element's latest official reference data separately.

No. Molar mass depends only on the types and counts of atoms present, so it calculates correctly without any charge information.
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Side Note — Why atomic weights aren't whole numbers

Carbon's atomic weight is 12.011, not a clean 12 — and the same is true for many elements. That's because most elements found in nature are mixtures of several isotopes (atoms with the same number of protons but a different number of neutrons), and the atomic weight is the abundance-weighted average of those isotopes' masses. Chlorine, for instance, is a mix of chlorine-35 (about 76% abundant) and chlorine-37 (about 24%), and that weighted average lands on the not-quite-round value of 35.45.

On the other hand, elements like fluorine (18.998) and sodium (22.990) sit very close to whole numbers — because in nature, those elements exist almost entirely as a single isotope, with essentially no mixing. The size of the fractional part in an atomic weight is actually a rough clue to how isotopically diverse an element is in nature.

The mole itself has an interesting history. It used to be defined as "the number of atoms in 12 grams of carbon-12," but the 2019 redefinition of the International System of Units (SI) fixed the Avogadro constant (about 6.022×10²³) itself as the defining value instead. That change made the mole's definition independent of any specific substance, including carbon-12.