In chemistry, the lever rule is a formula used to determine the mole fraction (x) or the mass fraction (w) of each phase of a binary equilibrium phase diagram. It can be used to determine the fraction of liquid and solid phases for a given binary composition and temperature that is between the liquidus and solidus line.
In an alloy or a mixture with two phases, α and β, which themselves contain two elements, A and B, the lever rule states that the mass fraction of the α phase is
where
*  is the mass fraction of element B in the α phase
*  is the mass fraction of element B in the β phase
*  is the mass fraction of element B in the entire alloy or mixture
all at some fixed temperature or pressure.