Module 2 of 4
The 21 Million Cap
How the supply cap works at the code level, why changing it is so hard, and what that means for monetary policy.
Film · Arithmetic everyone checks · 1:39
Block ———A short film for module 2 of Bitcoin as Money: fiat currency, with no fixed limit (US broad money since 1971); bitcoin's issuance schedule, 50 new bitcoin a block halving every 210,000 blocks; the cap as the sum of that schedule, just under 21 million; the last new bitcoin around 2140, by the schedule's arithmetic; the share already issued, counted live; 100 million sats to a bitcoin; every node checking every block; why changing the cap would make a separate coin; and fiat currency against bitcoin, where one bitcoin is always one bitcoin. It loops while on screen; use Pause to stop it. Turn on Narration to hear it explained aloud. A transcript follows.
Read the film, with its sources
- Bitcoin as Money · Module 2 of 4The 21 Million Cap. Bitcoin’s supply is capped at 21 million coins: the best-known fact about it, and one of the least understood. Where does that limit come from, and what stops anyone raising it?The Bitcoin Transition, the 21 million schedule (computed from the issuance rules)
- AUG 1971 → AUG 2026Start with what bitcoin is measured against. Fiat currency has no fixed limit: its supply grows as banks lend and as policy allows, and both can change. M2, the US broad money supply, was $685.5 billion in August 1971 and $23.3 trillion in August 2026: 34 times as much.Federal Reserve, H.6, via FRED (M2SL), aug 1971Federal Reserve, H.6, 22 September 2026, aug 2026
- The scheduleBitcoin’s limit comes from its issuance schedule. Each new block may create a set amount of new bitcoin, the block subsidy: 50 when the first block was mined in 2009. Every 210,000 blocks, about every four years, it halves: 25 from 2012, 12.5 from 2016, 6.25 from 2020, 3.125 from 2024. Dates from 2028 on are estimates.Bitcoin Core, src/validation.cpp, GetBlockSubsidy (read 16 Oct 2026)
- The sumAdd up the new bitcoin of every block and the total never quite reaches 21 million. The first 210,000 blocks created 10,500,000, half the total; the next era half as much again, and so on. Because each halving rounds down to a whole satoshi, the sum is 20,999,999.9769 bitcoin: just under 21 million. The cap is not set separately: it is the sum of the schedule.The Bitcoin Transition, the 21 million schedule (computed from the issuance rules)Bitcoin Core, src/validation.cpp, GetBlockSubsidy (read 16 Oct 2026)
- 2009 → 2140The subsidy keeps halving until it rounds down to zero, after 33 eras, at block 6,930,000. At about four years an era, that is around 2140: an estimate from the schedule’s arithmetic, not a date anyone has set. Most of the supply comes early; the last century adds only a sliver.The Bitcoin Transition, the 21 million schedule (computed from the issuance rules)
- Issued so farMore than 95 per cent of all the bitcoin there will ever be has already been issued: by block 924,000, mined on 17 November 2025, the schedule had issued 19,950,003 bitcoin. Anyone running the software can count them; the film shows the count at the latest block.Block 924,000, timestamp as recorded on the chain, via mempool.space (read 16 Oct 2026)The Bitcoin Transition, the 21 million schedule (computed from the issuance rules)
- SatsScarce does not mean too few to use. Each bitcoin is 100,000,000 satoshis, or sats, the unit the rules count in, so a bitcoin can be split into very small amounts.Bitcoin Core, src/consensus/amount.h, COIN (read 16 Oct 2026)
- Who enforces itWho enforces the cap? Every full node, a computer running bitcoin’s software, checks every block against the rules, the schedule included. A block whose miner pays itself more than the subsidy plus its transactions’ fees is invalid, and every node that checks it rejects it, whoever mined it.Bitcoin Core, src/validation.cpp, ConnectBlock, “bad-cb-amount” (read 16 Oct 2026)
- Could it change?Could the cap change? The software is open source, and anyone can propose new rules, but no one can make others run them. People who kept the existing rules would reject blocks that broke them, so a larger supply would make a separate coin, while bitcoin carried on with its 21 million. No one can print more than the schedule allows.The Bitcoin Transition, the 21 million schedule (computed from the issuance rules)
- Fiat currency and bitcoinFiat currency has no fixed limit: its supply grows as banks lend and policy allows. Bitcoin’s supply follows a fixed schedule that anyone can check. Measured in fiat, its price moves, but one bitcoin is always one bitcoin. Few prices are set in it yet, and keeping it safe takes care.
- The Bitcoin TransitionNot a promise. Arithmetic everyone checks.
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 separate rule. It is a consequence of the issuance schedule — the starting block subsidy and the halving interval — which Satoshi chose, in his own words, as "an educated guess", and which every node now enforces.
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.