In chemistry, the empirical formula of a chemical is a simple expression of the relative number of each type of atom (called a chemical element) in it. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. Empirical formulas are the standard for ionic compounds, such as CaCl₂, and for macromolecules, such as SiO₂. The term empirical refers to the process of elemental analysis, a technique of analytical chemistry used to determine the relative percent composition of a pure chemical substance by element.

In contrast, the molecular formula identifies the absolute number of atoms of such element to be found in each discrete molecule of that compound.

For example, hexane could have a structural molecular formula of CH₃CH₂CH₂CH₂CH₂CH₃, implying that it has a straight chain structure, 6 carbon atoms, and 14 hydrogen atoms. The hexane molecular formula is C₆H₁₄. And the empirical formula for the same molecule would be C₃H₇ showing a C:H ratio of 3:7.

In physics, an empirical formula is a mathematical equation that predicts observed results, but has no known theoretical basis to explain why it works.

An example was the Rydberg formula to predict the wavelengths of hydrogen spectral lines. Proposed in 1888, it perfectly predicted the wavelengths of the Lyman series, but until Niels Bohr produced his Bohr model of the atom in 1913, nobody knew why the formula worked.