The equilibrium constant expression is: , where is the coefficient in the chemical equation for the corresponding substance (positive for products and negative for reagents), and is the activity of the substance, i.e. the value that characterises the work expended in transferring a small part of the substance into the considered reaction system from a standard state.

For example, for substances forming immiscible pure phases, the activity is equal to one; in highly diluted solutions the activity is considered equal to concentration; in diluted gases, to partial pressure, etc. In equilibrium, the value , calculated using the above formula, should be a constant maintained (subject to changes of activity values) after the addition of any components and dependent only on temperature and (except for some model cases) pressure. If , the equilibrium is shifted toward the products; if , the equilibrium is shifted toward the reagents.