Molecular Weight Calculator

Molecular weight is the mass of one molecule, built by adding up the atomic masses of all its atoms. The same number is the molar mass: the grams in one mole of the substance. Water's molecular weight is 18.015, so one mole of water weighs 18.015 g.

Multiply each element's atom count by its atomic mass and add the results: M = Σ (atoms × atomic mass). For H2O that's 2 × 1.008 + 1 × 15.999 = 18.015 g/mol. This calculator shows every step in the breakdown table.

Only the units differ. Molecular weight is the mass of a single molecule in atomic mass units (amu); molar mass is the mass of one mole in grams per mole (g/mol). The number is identical, so chemists use the terms interchangeably.

From the standard atomic weights published by IUPAC — each is the weighted average of an element's naturally occurring isotopes. For synthetic elements with no stable isotopes, the table uses the mass number of the longest-lived isotope.

Usually rounding: 18.015 vs 18.02 g/mol for water is the same value with fewer decimals. Some elements (like carbon or lead) also have slightly different isotopic mixes in different samples, which shifts the average by a tiny amount.

Divide the grams by the molar mass: moles = grams ÷ g/mol. With 36.03 g of water, that's 36.03 ÷ 18.015 = 2 moles. This is the step that feeds into molarity calculations.