How many bitcoins can exist under Bitcoin’s original protocol limit?

Bitcoin’s original protocol limits the total supply to 21 million bitcoins.

New bitcoins enter circulation as block subsidies paid to miners that add valid blocks to the blockchain. Bitcoin’s protocol periodically halves this subsidy, reducing the rate at which new coins are created. With these scheduled reductions, the issuance approaches 21 million rather than continuing indefinitely.

The limit is not the same as the number of bitcoins currently circulating. Coins are released over time, and the final small fraction is expected to be issued around the year 2140 under the commonly cited schedule. Some bitcoins may also be inaccessible because their private keys have been lost, so the number available for spending can be lower than the number issued.

The 21-million figure is a rule enforced by Bitcoin’s software and network participants, not a physical resource limit. Changing it would require broad agreement among users, miners, and other participants running compatible software. The smallest unit, the satoshi, allows bitcoin to be divided into 100 million parts.