Why Bitcoin has a fixed supply

Many people hear that “there will only ever be 21 million bitcoins” and treat the phrase as a slogan. It is actually the result of a calculation. The system defines how many new coins each block can create and how that number shrinks over time. Add it all up and you get a maximum total.

This article explains where the cap comes from, how issuance works, who enforces the rule and what the cap does not guarantee. If you are not yet familiar with the basics of the network, start with what Bitcoin is.

How new bitcoins come into existence

There is no authority that “prints” money in Bitcoin. Satoshi Nakamoto’s whitepaper describes the solution: by convention, the first transaction in each block is special and creates new coins for whoever produced the block (Bitcoin whitepaper, section 6). This transaction is called the coinbase (not to be confused with the company of the same name).

The original paper compares the process to gold miners expending resources to add gold to circulation. In Bitcoin’s case, the resources spent are computing power and electricity. That is why those who produce blocks are called miners. Mining is explained in detail in how Bitcoin mining works.

On top of the new coins, the miner collects the fees paid by the transactions included in the block. That sum (new coins plus fees) is the block reward. The portion that comes from new coins is called the subsidy.

The issuance rule in the code

The subsidy is not decided block by block. It is calculated by a function in Bitcoin Core, the network’s reference software, called GetBlockSubsidy (Bitcoin Core, validation.cpp). The logic is short:

1. Divide the block height (its position in the chain) by the 210,000-block interval. The result is the number of “halvings” that have already occurred.
2. Start from 50 bitcoins and cut that amount in half once for each halving.
3. If the number of halvings reaches 64, return zero.

The 210,000-block interval is a main network parameter, defined as nSubsidyHalvingInterval (Bitcoin Core, chainparams.cpp). The same file sets the target time between blocks at 10 minutes. A comment in the code sums it up: the subsidy halves every 210,000 blocks, which happens “approximately every 4 years”.

The arithmetic is done in satoshis, Bitcoin’s smallest unit. One bitcoin equals 100 million satoshis, according to the COIN constant in the code (Bitcoin Core, amount.h). Working with whole numbers avoids rounding errors.

Why the sum comes to almost 21 million

Here are the first stages, each lasting 210,000 blocks (Bitcoin Wiki, “Controlled supply”):

Stage	Starting block	Subsidy per block	Coins created in the stage
1	0	50 BTC	10,500,000
2	210,000	25 BTC	5,250,000
3	420,000	12.5 BTC	2,625,000
4	630,000	6.25 BTC	1,312,500
5	840,000	3.125 BTC	656,250

Each stage issues half as much as the one before. It is a geometric progression: 10.5 million + 5.25 million + 2.625 million and so on. A sum of this kind never exceeds twice the first term, that is, 21 million.

In practice, the total ends up slightly lower. Because the subsidy is measured in whole satoshis, fractions are discarded at each cut. By the Bitcoin Wiki’s calculation, the theoretical maximum is 20,999,999.9769 bitcoins. Our newsroom redid the calculation from the rule in the code and arrived at the same number. The subsidy reaches 1 satoshi per block in stage 33 and zero in stage 34, starting at block 6,930,000.

The exact date this happens depends on the actual pace of blocks, which varies. The same page estimates that issuance will end around 2140, while noting that the forecast is difficult.

Where issuance stands today

The fourth halving took place at block 840,000, mined on April 20, 2024, according to the block data on the mempool.space explorer (block 840,000). Since then, each block has created 3.125 BTC.

On October 8, 2026, the chain was at block 970,493 (mempool.space, current height). Under the issuance rule, that corresponds to roughly 20.1 million bitcoins already created, about 95.7% of the maximum. This is our newsroom’s calculation based on block height; it includes coins that, as you will see below, can never be spent.

The next cut is scheduled for block 1,050,000. With blocks every 10 minutes, that would be about 550 days from October 2026, that is, around 2028. It is an estimate: the real date depends on the pace of blocks. The effects of the halving are covered in a separate article: the Bitcoin halving.

Who enforces the cap

The cap does not depend on a company or a promise. It is enforced by every node, the computer that runs Bitcoin software and checks blocks independently.

In Bitcoin Core, when validating a block, the node calculates how much the miner may receive: the fees from the block’s transactions plus the subsidy for that height. If the coinbase transaction pays more than that, the block is marked invalid with the reason bad-cb-amount, “coinbase pays too much” (Bitcoin Core, validation.cpp). A miner who tried to create extra coins would have the block rejected by the nodes that follow this rule, and would lose the work spent.

There is also a constant called MAX_MONEY, equal to 21 million bitcoins. The code comment itself warns that it is not the total supply but a sanity check: no amount above it is accepted (Bitcoin Core, amount.h). What limits issuance is the subsidy rule.

A technical detail fixed in 2014

In the original version, halving was done with a bit-shift operation. After 64 halvings, the behavior of that operation in C++ would become undefined, and the subsidy could, in theory, go back to 50 BTC around the year 2214. BIP 42, a 2014 Bitcoin Improvement Proposal authored by Pieter Wuille, proposed fixing the subsidy at zero from that point on (BIP 42). The document has a tongue-in-cheek tone, but the fix is in force: it is the line that returns zero when halvings reach 64.

Can the cap change?

Technically, the code can be changed, like any software. The question is whether the change would be accepted. The bitcoin.org FAQ states that the protocol cannot be modified “without the cooperation of nearly all its users”, and that changes only take effect if users, miners and node operators adopt them voluntarily (Bitcoin.org, FAQ).

In practice, a group that started accepting more issuance would create a separate network with different rules. Nodes that keep the original rule would keep rejecting those blocks. This is not a mathematical guarantee that nothing will change: it is a high social and technical barrier.

What the cap does not guarantee

The issuance ceiling is a technical rule. It says nothing about future value, and it is important to keep the two apart.

• Limited supply does not guarantee price. Price depends on demand, and demand can fall. Crypto assets can suffer steep and prolonged declines, and it is possible to lose everything invested.
• Lost coins are still counted. Bitcoins whose private keys have been lost sit idle forever, according to the bitcoin.org FAQ. They count toward the total issued but do not circulate. No one knows exactly how many there are.
• Some coins could never be spent. The 50 BTC from the network’s first block (the genesis block) cannot be spent. The Bitcoin Wiki also records cases of miners who claimed less than they could and coins destroyed on purpose. That is why the effective supply is below the theoretical maximum.
• Scarcity is not stability. The bitcoin.org FAQ says Bitcoin “is not designed to be a deflationary currency”. Having a predictable supply does not make the price predictable.

The question of fees

There is also a long-term issue. The whitepaper anticipates that once a predetermined number of coins have entered circulation, the incentive for miners can transition entirely to transaction fees.

Today, fees are still a small part of the reward. In the 1,008 blocks between blocks 969,480 and 970,487 (about one week, through October 8, 2026), fees totaled about 19.3 BTC, against roughly 3,169 BTC in total rewards, or about 0.6% (mempool.space, reward statistics). This percentage varies widely with demand for block space.

Whether fees will be enough to pay for the network’s security when the subsidy becomes very small is an open question. In our newsroom’s assessment, anyone claiming to have the definitive answer is speculating.

Key takeaways

• The cap of almost 21 million is not a promise: it is the sum of an issuance that starts at 50 BTC per block and halves every 210,000 blocks.
• The theoretical maximum is 20,999,999.9769 BTC, and issuance should end around 2140. By October 2026, about 95.7% had already been created.
• Every node on the network rejects blocks that pay more than allowed. Changing the rule would require broad support from those who use and run the network.
• Limited supply does not guarantee price, stability or returns. Lost coins, the future reliance on fees and volatility are real risks.