What is 21 million cap?
Bitcoin's maximum supply, which emerges from the halving schedule rather than being set as a separate parameter.
Sum the block subsidy across all halving epochs and the series converges just under 21 million. The cap is not a rule someone could edit in isolation; it is a consequence of the issuance schedule every node enforces.
The final fraction will be issued around 2140, though the practical effect of issuance ends far earlier: over 99% will have been mined decades before.
Related terms
Halving
The scheduled 50% reduction of the block subsidy, occurring every 210,000 blocks — roughly every four years.
EconomicsBlock subsidy
The newly issued bitcoin paid to the miner of each block. It began at 50 BTC and halves every 210,000 blocks.
On-chain dataCirculating supply
The quantity of bitcoin that has been mined, verifiable directly against the issuance schedule.
EconomicsScarcity
Bitcoin's fixed and verifiable supply limit, enforced by every node rather than promised by an issuer.
This definition is part of the Bitcoin Logical glossary. For the fuller explanation, start with Обучение, or see the live numbers on Сигнал Bitcoin.