Bitcoin's hard-coded maximum supply of 21 million coins. This fixed limit, enforced by the protocol's consensus rules, makes Bitcoin the first provably scarce digital asset.
Bitcoin's hard-coded maximum supply of 21 million coins. This fixed limit, enforced by the protocol's consensus rules, makes Bitcoin the first provably scarce digital asset.
The 21 million supply cap is one of Bitcoin's most important properties. It is embedded in the protocol's code and enforced by every node on the network. The cap is achieved through the halving mechanism: the block reward starts at 50 BTC and halves every 210,000 blocks. The geometric series 50 + 25 + 12.5 + 6.25 + ... converges to exactly 21 million, meaning no more than 21 million Bitcoin can ever exist.
As of 2024, approximately 19.7 million Bitcoin have been mined, leaving only about 1.3 million to be created over the next century-plus. The issuance rate is currently 3.125 BTC per block (after the April 2024 halving) and will continue decreasing until the last satoshi is mined around the year 2140. This predictable, disinflationary supply schedule is a key differentiator from fiat currencies, whose supply can be expanded indefinitely by central banks.
The supply cap is also why Bitcoin is often called "digital gold" or "sound money." Just as gold's scarcity underpins its value as a store of wealth, Bitcoin's mathematically guaranteed scarcity creates a foundation for long-term value accrual. Unlike gold, however, Bitcoin's supply schedule is perfectly known in advance, and its total supply can be verified by anyone running a node. An estimated 3-4 million Bitcoin are also permanently lost (inaccessible private keys), making the effective circulating supply even lower than 21 million.
Technically, the supply cap is a parameter in Bitcoin's code that could be modified. However, doing so would require a hard fork that the overwhelming majority of nodes, miners, and users would need to accept. Since the supply cap is considered Bitcoin's most sacred property — the foundation of its value proposition — any attempt to change it would almost certainly be rejected by the network and would likely result in a chain split where the "real" Bitcoin remains the version with 21 million coins.
Approximately 1.3 million BTC remain to be mined as of 2024. However, the rate of new issuance slows dramatically with each halving. About 90% of all Bitcoin has already been mined, and 99% will have been mined by approximately 2035. The final Bitcoin will not be mined until around 2140 due to the exponentially decreasing block reward.
After the last Bitcoin is mined (around 2140), miners will earn revenue solely from transaction fees. By that point, the block reward will have been negligible for decades, and the fee market will have had over a century to develop. If Bitcoin is widely used as a store of value and settlement layer, transaction fees should provide sufficient incentive for miners to continue securing the network.