In chemistry, the empirical formula of a chemical is a simple expression of the relative number of each type of atom (element) in it. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. The term empirical refers to the process of elemental analysis, an analytical technique used to determine the relative percent composition of a pure chemical substance by element.

For example, hexane could have a chemical formula of CH₃CH₂CH₂CH₂CH₂CH₃, implying that it has a straight chain structure, 6 carbon atoms, and 14 hydrogen atoms. However the empirical formula for the same molecule would be C₃H₇.