Formula mass is the mass of one formula unit of a substance, equal to the sum of the atomic (or ionic) masses of the atoms in its formula. The term is used in place of molecular mass for ionic substances, which do not exist as discrete molecules. For example, the formula mass of sodium chloride is the sum of the masses of one Na⁺ and one Cl⁻.

式量是物质一个式单元的质量,等于该式中各原子(或离子)质量之和。对于不以独立分子形式存在的离子化合物,使用这一术语来替代分子质量。例如,氯化钠的式量是一个 Na⁺ 与一个 Cl⁻ 质量之和。