Why Is There a Cap of 21 Million Bitcoin?

The short answer

Bitcoin only issues new units through the block reward, the amount a miner gets for finding a block. The GetBlockSubsidy function in Bitcoin Core says: the reward starts at 50 bitcoin per block and halves every 210,000 blocks, which works out to roughly every four years.

The cap comes out of that chain. 50 plus 25 plus 12.5 and so on adds up to just under 100 bitcoin. Since each step lasts 210,000 blocks, that makes just under 21 million bitcoin in total. After enough halvings the reward hits zero, and from then on no new bitcoin are created.

The halving schedule in detail

The GetBlockSubsidy function works the reward out like this: take the block number, divide it by 210,000, and halve the starting reward of 50 bitcoin that many times. At 64 or more halvings the code pins the reward to zero.

• The reward is held internally in satoshis, as whole numbers. 50 bitcoin is 5,000,000,000 satoshis.
• Because it keeps halving and rounds down, the reward is still one satoshi after 32 halvings and drops to zero with the 33rd, at block 6,930,000. On the current block pace that lands around the year 2140.
• How a block reward comes about and what it is for is covered in what is Bitcoin mining.

So the schedule is fully known in advance. You can work out how many new bitcoin any future block will pay without asking anyone.

Why the real total is under 21 million

21 million is a round figure. The Bitcoin Core source code is more careful here. It holds a value called MAX_MONEY, 21,000,000 times 100 million satoshis, and the comment beside it says plainly: this is a sanity check, not a statement about the actual money supply. The amount that will ever really exist is a little below that, for several technical reasons.

If you want to be precise, then, it is a ceiling rather than a final total. The limit is never quite reached, and under the rules it is never exceeded.

Could anyone raise the cap?

The cap is a consensus rule. Every full node checks, on every block, that no oversized reward is paid out, and rejects a block that breaks that. Pushing through a higher reward would mean practically all of those checkers running changed software at the same time.

That is not something one group decides. A rule change of this size would need a lot of independent parties behind it: the operators of economically important nodes, wallets and services, the users, and the miners. Anyone who runs a different rule without the other parties following ends up on a forked chain. It is exactly this high bar that makes the 21 million count as stable.

Why the supply is capped at all

The Bitcoin whitepaper describes the block reward as an incentive: it brings computing power to the network and puts new bitcoin into circulation without a central body having to issue them. The reward drops on schedule, and once enough bitcoin are in circulation, miners are funded purely by transaction fees. The whitepaper explicitly calls that state free of inflation in the sense of a growing money supply.

Whether a fixed cap is economically good or bad is a matter of debate, and not the point of this article. What can be said is only this: it is part of the rules, known in advance, and not changeable at short notice. Why Bitcoin exists in the first place is covered in the basics section.