Molar mass is a term used in chemistry to describe the mass of a given chemical element or compound divided by the amount of substance measured in moles. It is defined as the mass of a substance per mole of that substance. The molar mass is a bulk property of a substance, not a molecular property. It is an average of many instances of the compound, which often vary in mass due to the presence of isotopes. The molar mass is calculated as the sum of standard atomic weights of all atoms that form one entity of the substance.

Molar mass is an intensive property of the substance, meaning that it does not depend on the size of the sample. It is used as a bridge between the mass of a material and the number of moles since it is not possible to measure the number of moles directly. The molar mass is appropriate for converting between the mass of a substance and the amount of a substance for bulk quantities.

The molecular mass and formula mass are commonly used as synonyms of molar mass, particularly for molecular compounds. However, the most authoritative sources define them differently. The difference is that molecular mass is the mass of one specific particle or molecule, while the molar mass is an average over many particles or molecules. The formula weight is a synonym of molar mass that is frequently used for non-molecular compounds, such as ionic salts.

Molar masses are almost never measured directly. They may be calculated from standard atomic masses, and are often listed in chemical catalogues and on safety data sheets. The coherent unit of molar mass in the International System of Units (SI) is kg/mol, but for historical reasons, molar masses are almost always expressed in g/mol.