Glossary / Basics
21 million supply cap
Also known as Hard cap, Supply cap, 21 million.
- Definition
- The 21 million supply cap is bitcoin's permanent ceiling on how many coins can ever exist, enforced by every node through a halving schedule no participant can override.
New coins enter circulation only as a block subsidy, and that subsidy halves every 210,000 blocks until it rounds to zero. The subsidy fell to 3.125 bitcoin at block 840,000 on April 20, 2024, and roughly 20 million of the 21 million have been issued. For a buyer, the cap is why bitcoin is treated as a scarce asset rather than a currency whose issuer can print more of it.
How it works
Nothing about the 21 million cap is a promise. It is arithmetic that every node rechecks on every block it receives. A block may create new coins in exactly one place, its coinbase transaction, and only up to the amount the rules permit at that height. Claim one satoshi more and the block is rejected, no matter who mined it or how much electricity went into it.
The permitted amount began at 50 bitcoin per block in January 2009 and is cut in half every 210,000 blocks, which works out to roughly four years. Because satoshis are whole numbers, the halving eventually has nothing left to divide: the subsidy shifts to zero at the thirty-third halving, expected around 2140. Summing the entire series gives 20,999,999.9769 bitcoin rather than a round 21 million, because the integer arithmetic truncates at several steps along the way.
The total can only fall from there. Coins locked behind lost keys still count toward the 21 million but will never move again, and a small number have been provably destroyed. Two coinbase transactions in late 2010 were byte-for-byte duplicates of earlier ones, which quietly erased 100 bitcoin from the spendable supply and prompted the BIP30 rule forbidding duplicate coinbase transactions. Nobody can mint replacements for any of it.
Why this matters when you buy bitcoin
A fixed supply changes what a purchase is, not what it is worth. Scarcity sets no price, and the market has cut bitcoin's dollar value by more than half several times while the issuance schedule ticked along untouched. What the cap tells you is that dilution will not be the thing that hurts you. It says nothing whatsoever about the next year of demand.
What the cap does change is the cost of accumulating. Issuance now runs at about 450 new coins a day, well under one percent a year against the coins already circulating, so nearly everything you buy comes from another holder rather than from a miner. That makes your fee drag the variable you actually control. The platforms reviewed here differ enormously on this point: River charges no platform fee on recurring buys, Swan Bitcoin charges a flat fee of about 0.99 percent per purchase, and Kraken Pro starts at 0.25 percent maker and 0.40 percent taker below $10,000 of 30-day volume, while Kraken's simple Buy widget costs considerably more for the identical coins.
The cap is also the one bitcoin fact that no government can touch. Rules across the 231 countries covered on this site decide whether you may buy, from whom, and what you declare at tax time. None of them changes how many coins exist. A ban shrinks your access to the market, not the ledger.
The issuance schedule, halving by halving
Each era mints 210,000 blocks at its own subsidy, which makes the whole history auditable on a single screen:
- Blocks 0 to 209,999: 50 bitcoin each, 10,500,000 coins, beginning January 3, 2009.
- Block 210,000, November 28, 2012: 25 bitcoin each, 5,250,000 coins.
- Block 420,000, July 9, 2016: 12.5 bitcoin each, 2,625,000 coins.
- Block 630,000, May 11, 2020: 6.25 bitcoin each, 1,312,500 coins.
- Block 840,000, April 20, 2024: 3.125 bitcoin each, 656,250 coins.
- Block 1,050,000, expected in 2028: 1.5625 bitcoin each.
Two facts fall out of that list. The first era alone created half of every bitcoin that will ever exist, and it was finished before December 2012. And the coins left after any halving come to exactly twice what the new era will produce: 1,312,500 remained the moment block 840,000 was mined, a shade over 6 percent of the cap, spread across the century that follows.
21 million supply cap vs market capitalization
The supply cap counts coins and market capitalization counts dollars. Market cap multiplies the current price by the coins already issued, so it moves every second the price moves, while the cap has not changed since the software was published. The two get blurred in headlines about bitcoin's "cap", and in fully diluted valuations that multiply today's price by the full 21 million as though the unmined coins were already for sale.
21 million supply cap vs the halving
The halving is the mechanism and the cap is the consequence. A halving is a single scheduled event, every 210,000 blocks, that cuts the block subsidy in two; the cap is the sum of that shrinking series. Hitting the cap will not stop mining. It means miners get paid by transaction fees alone, a future already glimpsed at block 840,000 itself, which paid its miner about 37.6 bitcoin in fees against a 3.125 bitcoin subsidy.