Explanation
The total supply of BTC is limited and pre-defined in the Bitcoin protocol at 21 million, with the mining reward (how Bitcoins are created) decreasing over time. This graph shows how many Bitcoins have already been mined or put in circulation.
Notes
The Bitcoin reward is divided by 2 every 210,000 blocks, or approximately four years. Some of the Bitcoins in circulation are believed to be lost forever or unspendable, for example because of lost passwords, wrong output addresses or mistakes in the output scripts.