Why are there 21 million bitcoins?

When all mining is done, there will be a maximum of 21 million bitcoins in circulation. Why was this number chosen? My speculation: 21 million was chosen to ensure a sensible smallest unit if Bitcoin were the world’s exclusive currency.

The real unit of Bitcoin is not the bitcoin; it is the satoshi: the satoshi is the unit used in transactions, and it is indivisible. There will be 2.1e15 satoshis in circulation. The indivisible unit in USD is the cent; it’s not really possible to sell items for fractions of a cent. There are about 6e15 cents in circulation today (across all currencies). Thus if everyone used Bitcoin, the satoshi would be worth about 3 cents, which the world has decided is a sensible minimum amount.

At this time, 1 BTC would be worth $2,800,000.

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