What is Stock-to-Flow?
When a Bitcoin miner satisfies a proof of work requirement with computational power and electricity, he or she is rewarded with a newly created bitcoin. The number of coins rewarded for a miner meeting this requirement began with fifty. It is halved every 210,000 blocks, which occurs roughly every four years. This will continue until the stock of existing bitcoins reaches 21 million, at which point, no more coins can be mined.
The stock-to-flow ratio is the ratio of current stock of bitcoin over incoming coins for a specific period. That means