The Stock to Flow (S/F) Ratio is a popular model that assumes that scarcity drives value. Stock to Flow is defined as the ratio of the current stock of a commodity (i.e. circulating Bitcoin supply) and the flow of new production (i.e. newly mined bitcoins). Bitcoin's price has historically followed the S/F Ratio and therefore it is a model that can be used to predict future Bitcoin valuations. This metric was first coined by PlanB.