A means to measure the abundance of a particular resource. It’s determined by the amount of a resource held in reserves divided by the amount that is produced annually. The Stock to Flow ratio essentially shows how much supply enters the market each year for a given resource relative to the total supply. The model has historically been applied to gold, but recently has been applied to bitcoin. Advocated of the model claim that BTC is a resource comparable to gold in that it’s scarce, costly to produce, and its maximum supply is capped at 21 million coins.