The quantity theory of money is a theory that variations in price relate to variations in the money supply. It is most commonly expressed and taught using the equation of exchange and is a key foundation of the economic theory of monetarism.

It argues that an increase in money supply creates inflation and vice versa.

The Irving Fisher model is most commonly used to apply the theory. Other competing models were formulated by British economist John Maynard Keynes, Swedish economist Knut Wicksell, and Austrian economist Ludwig von Mises.