Issued supply is a protocol quantity
Each valid block may contain no more than the permitted subsidy plus transaction fees. A full node derives the maximum subsidy from block height and verifies that the coinbase transaction did not create more. Issued units can therefore be derived from the public history.
Roughly every four years is only a calendar shorthand. A halving occurs exactly every 210,000 blocks, while block times vary. Repeated integer division ultimately takes the subsidy to zero and leaves total issuance just below 21 million.
Circulating supply depends on a definition
Market-data providers may subtract provably unspendable or estimated non-tradable units from issued supply. Their scope differs by methodology. Bitcoin has no registry marking a coin as active, lost or long-term savings.
An output can remain still because its owner is saving, waiting, deceased without a plan or has lost the key. The chain cannot reliably distinguish those stories. Any estimate of lost supply must disclose its assumptions and uncertainty.
Market availability changes without issuance
Some owners offer bitcoin on exchanges, others hold it away from markets, and some would sell only at another price. Liquid supply is therefore smaller and more variable than issued BTC. A transfer to an exchange does not prove an intention to sell, nor does a withdrawal prove permanent holding.
One unit can affect price repeatedly when traded many times; an inactive unit may have no effect on today's price. Total supply alone cannot reveal current selling pressure.
- issued supply is verifiable
- loss of a key is usually not provable
- tradable supply depends on owners and price
How to read supply metrics
First ask whether a metric uses issued, circulating, free-float or recently active supply. Then check how it treats old outputs, exchanges, custodians and known unspendable scripts. Two honest methodologies can produce different figures.
The protocol limit and specific output history are precise. Who economically controls an output, whether a backup still works and at what price its owner would trade are uncertain. Good analysis admits that boundary instead of publishing a falsely exact number of lost bitcoins.