Charles Law may refer to:

• Charles's law, also known as the law of volumes, experimental gas law which describes how gases tend to expand when heated
• Charles Law (British politician) (1792–1850), British judge and Conservative Party MP
• Charles B. Law (1872–1929), United States Representative from New York

Charles' law (also known as the law of volumes) is an experimental gas law which describes how gases tend to expand when heated. A modern statement of Charles' law is:

When the pressure on a sample of a dry gas is held constant, the Kelvin temperature and the volume will be directly related.

this directly proportional relationship can be written as:

or