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Why only 21 million?
Bitcoin's supply is capped by its code. About 96% of all bitcoin already exists.
- There will never be more than 21 million bitcoin.
- New coins come out on a fixed schedule that slows down every four years.
- More than 20.1 million already exist. The last fraction arrives around 2140.
New bitcoin comes only as a reward to miners. Miners are the computers that add a new batch of transactions, called a block, about every 10 minutes. The reward started at 50 BTC per block and is cut in half every 210,000 blocks, an event called the halving.
Add up a reward that keeps halving and it gets closer and closer to 21 million without ever passing it. No central bank or company can decide to print more, because every node rejects a block that pays out more than the rules allow.
That fixed supply is why many people compare bitcoin to gold, and why some call it "hard money."
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Could the cap ever change?
In theory, only if nearly everyone running Bitcoin chose new software that allows it. In practice, the cap is the main reason people hold bitcoin, so there is no support for raising it.
Is it exactly 21 million?
Slightly less. Because of rounding in the code, the true maximum is about 20,999,999.98 BTC. Some coins are also lost for good when people lose their keys, so the usable supply is lower still.
What pays miners after 2140?
Transaction fees. Fees already make up part of every miner's income, and the plan is for them to carry the full load once new coins stop.