What is molecular weight?
Molecular weight (also called molecular mass or molar mass) is the sum of the atomic masses of every atom in a molecule. It is expressed in atomic mass units (amu) or, equivalently, in grams per mole (g/mol). For example, water (H₂O) has a molecular weight of about 18.015: two hydrogen atoms at 1.008 amu each plus one oxygen atom at 15.999 amu. Knowing the molecular weight of a substance is essential for converting between grams and moles—the foundation of stoichiometry.
How to use this calculator
Type a chemical formula into the input field using standard notation: capitalise the first letter of each element symbol and add subscript numbers after it (e.g. H2O, CO2, C6H12O6). Quick-pick buttons for common compounds save typing. Click Calculate to see the total molecular weight, a row-by-row breakdown of each element's contribution, and a coloured percent-composition bar so you can visualise how much each element contributes to the total mass.
Molecular weight vs molar mass
The two terms are numerically identical but use different units. Molecular weight is stated in amu (or daltons), while molar mass is stated in g/mol. One mole of any substance contains Avogadro's number (6.022 × 10²³) of molecules and weighs exactly its molar mass in grams. This calculator displays both units for convenience.
Practical applications
Chemists use molecular weight to prepare solutions of a specific molarity, balance equations, and calculate yields. Pharmacologists determine drug dosages based on molar mass. Biochemists characterise proteins and nucleic acids by their molecular weights. Environmental scientists convert pollutant concentrations between mass-based and mole-based units. In every case, the calculation starts with the same simple sum of atomic masses.
Tips
Element symbols are case-sensitive—"Na" is sodium, but "NA" would be parsed as nitrogen + an unknown "A." This calculator supports simple formulas without parentheses; for hydrates or complex salts, expand them first (e.g. write CaSO4 as Ca S O4 separately). Atomic masses are IUPAC standard values. This free tool runs entirely in your browser—no sign-up, no download, instant results on any device. Bookmark it for quick molecular-weight lookups.