In quantum physics, the rule that gives measurement probabilities from a wave function is the Born rule.
Max Born proposed the rule in 1926. It states that the probability density for finding a particle at a particular position is proportional to the squared magnitude of its wave function, written |ψ|². More generally, the squared magnitude of a probability amplitude determines the chance of an associated measurement outcome.
A wave function itself is generally complex-valued, so it cannot be read directly as an ordinary probability. Squaring the magnitude produces a real, nonnegative quantity. The wave function must also be normalized so that the probabilities of all mutually exclusive outcomes add to one.
The Born rule is one of the basic postulates of standard quantum mechanics. It is not the same as the Schrödinger equation: the Schrödinger equation describes continuous state evolution, while the Born rule connects the mathematical state to observed measurement statistics. The rule is also central to quantum information theory.