Molecular Weight Calculator

Molecular weight (MW) is technically the mass of a single molecule in atomic mass units (amu or Da). Molar mass is the mass of one mole of molecules in grams per mole (g/mol). Numerically they are equal: water has MW = 18.015 amu and molar mass = 18.015 g/mol. In practice, chemists use the terms interchangeably.

Identify each element and count its atoms, then multiply the count by the atomic mass from the periodic table. Sum all contributions. For H2SO4: (2 x 1.008) + (1 x 32.06) + (4 x 15.999) = 2.016 + 32.06 + 63.996 = 98.072 g/mol. The atomic mass of each element is on the periodic table, usually below the element symbol.

A dalton (Da), also called an atomic mass unit (amu), is defined as 1/12 the mass of a carbon-12 atom = 1.66054 x 10^-24 g. Numerically, 1 g/mol = 1 Da per molecule when comparing mass scales. Biochemists commonly use kDa (kilodaltons) for protein masses - a protein with MW 50,000 Da = 50 kDa.

Every molar mass depends on the atomic-mass values used. This calculator uses IUPAC standard atomic weights (for example C = 12.011, O = 15.999, H = 1.008), which gives ethanol = 46.069 g/mol and CaCO3 = 100.086 g/mol. Some references round differently or use conventional values, so you may see ethanol quoted as 46.07 or CaCO3 as 100.09. These small differences come from rounding the atomic weights, not from an error, and are well within the natural uncertainty of the values.