Why 21 million.
The supply cap is the most famous property of Bitcoin and the least understood. It is not a policy. It is not a target. It is a mathematical consequence of the issuance schedule.
Just under 21 million BTC: each halving rounds down.
How many bitcoins are there right now?
95.68% of the cap
Issued over the next century or so
Block 1,050,000, around April 2028 at the recent pace
Live from the chain: Block 969,911. Computed from the issuance schedule; updates with each new block.
How the cap emerges
When Bitcoin launched in 2009, each new block rewarded the miner with 50 BTC. Every 210,000 blocks — approximately every four years — that subsidy halves. It dropped to 25 in 2012, 12.5 in 2016, 6.25 in 2020, and 3.125 in April 2024. The next halving will reduce it to 1.5625 in 2028.
If you sum the total bitcoin that can ever be issued under this schedule — 50 × 210,000 + 25 × 210,000 + 12.5 × 210,000 + ... — you converge on 20,999,999.9769. For all practical purposes, 21 million.
Who enforces it
Every full node on the Bitcoin network validates every block against the protocol rules, including the issuance schedule. A miner who tries to create a block with more than the allowed subsidy will have that block rejected by every node that sees it. No authority is required to enforce the cap. The cap is enforced by thousands of independent computers, each running its own verification.
Why changing it is so hard
In principle, the software could be changed. In practice, a change would require every node operator to voluntarily install the new software and accept the new rules. They would not. Holders of bitcoin have no incentive to dilute their own holdings; node operators have no incentive to run software that undermines what they already hold.
Past attempts to change Bitcoin's core properties have resulted in forks — new networks that diverged from the main chain. Each of these forks has lost value steadily relative to the original Bitcoin chain. The market has repeatedly selected the chain with the unchanged rules, and there is no reason to expect a different outcome from a hypothetical future attempt.
What happens after 2140
Around the year 2140, the block subsidy will round to zero. From that point onward, miners will be compensated exclusively by transaction fees paid by users. The network will continue to process transactions and maintain security, but no new bitcoin will be created at the protocol level. The supply becomes fixed, permanently.