Module 2 of 4
The 21 Million Cap
How the supply cap works at the code level, why it cannot be changed, and what that means for monetary policy.
Bitcoin's supply is capped at 21 million coins. This is the most commonly cited fact about Bitcoin, and one of the least understood. The cap is not a number a developer picked. It is a consequence of the issuance schedule, the block subsidy, and the halving interval, all of which are encoded in the protocol.
How the Cap Emerges
When a miner successfully produces a new block, the protocol allows them to include a transaction — the coinbase transaction — that creates new bitcoin from nothing. The amount created is called the block subsidy. At Bitcoin's launch in 2009, the subsidy was 50 BTC per block.
Every 210,000 blocks — approximately every four years — the subsidy halves. It dropped to 25 BTC in 2012, 12.5 BTC in 2016, 6.25 BTC in 2020, and 3.125 BTC in 2024. It will continue halving until the subsidy rounds to zero, around the year 2140.
If you sum the total bitcoin that can ever be issued under this schedule, the answer is approximately 20,999,999.9769 — essentially 21 million. The cap is not an arbitrary ceiling. It is the mathematical limit of an infinite geometric series.
Who Enforces It
The supply cap is enforced by every full node on the Bitcoin network. Each node validates every transaction against the protocol rules, including the issuance schedule. A miner who tries to include more than the allowed block subsidy in their block will have that block rejected by every node that sees it. The invalid block never enters the chain.
This is worth stating precisely. There is no authority that enforces the cap. There are thousands of independent computers, run by people who have never met, each running software they downloaded themselves, each independently checking every block. A change to the supply schedule would require every one of them to install new software that accepted the new rules. They would not do it.
This is the practical meaning of "decentralised". Not that no one is in charge, but that no one is in charge in a way that matters for the supply cap.
Could It Ever Change?
In principle, yes. The Bitcoin protocol is open-source software. Anyone can propose a change. The question is whether the proposed change would be accepted by the users running the network.
A proposal to increase the supply cap would split the network. The people who oppose the change would continue running the existing software and would reject blocks that violated the old rules. The people who accepted the change would be running a different network, with a different history, and a different asset. The original Bitcoin — with the 21 million cap — would continue as before.
This has happened several times, most notably with Bitcoin Cash in 2017. The Cash fork changed several rules. It became a different asset, trading at a fraction of Bitcoin's value, and has continued to lose ground ever since. Bitcoin, with its original rules, continued.
The cap is as firm as any man-made rule can be. It is enforced not by threat of punishment but by the collective refusal of thousands of independent operators to run different software. That is stronger than any legal guarantee.