Molecular weight calculator

How molar mass is worked out

Molar mass, often called molecular weight, is the mass of one mole of a substance in grams. For each element in the formula, multiply its standard atomic weight by the number of atoms, then add the results. For water, H₂O: 2 × 1.0080 + 15.999 = 18.015 g/mol. The table under the answer shows each element's share, so you can check every line.

Writing the formula

Start each element with a capital letter, then at most one small letter: Cl, not CL. This matters because Co is cobalt but CO is carbon and oxygen. A number after an element or a bracket multiplies it: Ca(OH)₂ has two O and two H. Round, square and curly brackets all work and can be nested, as in K₄[Fe(CN)₆].

For a hydrate, put a dot before the water: CuSO₄·5H₂O. You can type the dot as *, a full stop or ·. A number at the start of a part multiplies that part.

The atomic weights

The page uses the abridged standard atomic weights published by IUPAC's Commission on Isotopic Abundances and Atomic Weights (CIAAW), 2024 edition, listed in full below. Results are shown to three decimal places. Elements with no stable isotopes, such as technetium and plutonium, have no standard atomic weight, so the page will not guess one.

Charges, states such as (aq) and isotope labels are not read.