Every block pays its producer a subsidy. The subsidy began at 50 BTC and halves every 210,000 blocks. With a ten-minute block target, 210,000 blocks take roughly four years.
The sum of 210,000 × 50 + 210,000 × 25 + 210,000 × 12.5 … converges. Because the protocol counts in satoshis using integer arithmetic, the halvings eventually round the subsidy down to zero, and total issuance ends slightly below 21,000,000 BTC.
The 21 million figure is therefore a consequence of the rules, not a target somebody has to defend. Changing it would require the people running nodes to adopt software with different rules, and nodes that kept the old rules would simply reject the new blocks.
Explain more simply
New bitcoin is created as a reward for adding a block. That reward started at 50 and gets cut in half roughly every four years.
Halve something forever and the total stays finite. Add all the rewards together and you get just under 21 million.
Real-world analogy
Imagine a spring that releases half as much water every four years. It never fully stops on a specific announced day, it simply becomes too small to matter, and then it is empty.
Key facts
- Subsidy halves every 210,000 blocks.
- The ten-minute block interval is an average target, maintained by difficulty adjustments every 2,016 blocks.
- After the subsidy reaches zero, miners are paid only by transaction fees.
Common misconception
“Developers could just raise the 21 million limit.”
Developers can publish any software they like. Whether it becomes Bitcoin depends on nodes, users, exchanges and businesses choosing to run it. Nodes running the existing rules reject blocks that create extra coins.[1]
Go deeper
In Bitcoin Core the subsidy is computed as 50 × 100,000,000 satoshis shifted right by the halving count, so era 33 onwards yields sub-satoshi amounts that truncate to zero.
A handful of early blocks paid less than the maximum allowed subsidy because of implementation quirks in mining software, so the real final supply is marginally lower than the theoretical sum. This is observable in the chain data, not a matter of opinion.
Quick check
Answer every question correctly (100%) to complete this lesson.
1.How often does the block subsidy halve?
2.Who enforces the supply limit?
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Extra exam questions
Every question here counts towards your accuracy, XP and rank. No guessing: every answer is explained.
Quick check
Answer every question correctly (100%) to complete this lesson.
1.What is the maximum number of bitcoin that will ever exist?
2.How is the 21 million cap enforced?
3.What mechanism gradually reduces new bitcoin issuance over time?
4.Approximately how often does a halving occur?
5.Because of rounding in whole satoshis, when will the last new bitcoin actually be issued?
6.Can the 21 million cap be raised by a group of miners deciding to change it?
7.What is a satoshi in relation to the 21 million cap?
8.Why is a fixed, known supply cap considered significant for bitcoin?
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Sources
- Bitcoin: A Peer-to-Peer Electronic Cash System (2008), Satoshi Nakamoto
The original nine-page proposal. Describes proof of work, timestamping and the incentive model.
- Bitcoin Core documentation, Bitcoin Core project
Reference implementation documentation, including validation and release notes.