Bitcoin Block 600,000 Put the 18 Million Supply Milestone on the Ledger

Bitcoin reached block height 600,000 at 00:04:21 UTC on October 19, 2019, adding another proof-of-work block to a chain that had been running for more than a decade. The block’s hash was 00000000000000000007316856900e76b4f7a9139cfbfba89842c8d196cd5f91.

The milestone was widely described at the time as the moment the 18 millionth bitcoin had been mined. That shorthand captured the broad monetary-policy story, but it blurred three different measures: the maximum subsidy permitted by the protocol, the amount miners actually claimed, and the amount economically spendable or circulating.

What block 600,000 recorded

Mempool.space’s reading of the Bitcoin ledger reports 1,925 transactions in the block, a size of 870,371 bytes and a weight of 2,848,472 weight units. Its coinbase transaction claimed 12.53764047 BTC: a 12.5 BTC block subsidy plus 0.03764047 BTC in transaction fees.

Those figures describe one block at one height. They are not estimates of daily network volume, and transaction output value should not be treated as money changing beneficial owners because Bitcoin transactions can include change returned to a sender. The timestamp is also a miner-supplied header value accepted under Bitcoin’s consensus rules, not an external atomic-clock certification, although independent explorers reproduce the same October 19 time and block hash.

The block did not change Bitcoin’s issuance rules. Bitcoin Core’s contemporaneous version 0.18.1 set the main-network subsidy-halving interval at 210,000 blocks. Its subsidy function began from 50 BTC and shifted that amount down by one half for each completed interval. At height 600,000, the permitted subsidy therefore remained 12.5 BTC; the next reduction was scheduled by block height, not by a fixed calendar date.

The 18 million figure needs a label

A simple schedule calculation explains the headline. Heights 0 through 209,999 permitted 50 BTC each, heights 210,000 through 419,999 permitted 25 BTC each, and heights 420,000 through 599,999 permitted 12.5 BTC each. On that theoretical maximum schedule, cumulative subsidies reached exactly 18 million BTC after block 599,999; block 600,000 raised the total including the genesis subsidy to 18,000,012.5 BTC.

That calculation is interpretation derived from the code and heights, not a separate field stored in block 600,000. It also is not identical to spendable supply. Bitcoin Core’s source notes that the genesis block’s 50 BTC output cannot be spent, and miners can claim less than the maximum subsidy. Coins whose keys have been lost remain on the ledger but may be economically inaccessible. “Circulating supply” therefore depends on a definition that the block itself does not provide.

Contemporaneous coverage on October 19 reported that mined supply had crossed 18 million and emphasized that only about 3 million of the nominal 21 million remained. The defensible event-day conclusion is narrower: the chain passed height 600,000 at the point its programmed subsidy schedule was around the 18 million mark. It did not establish that exactly 18 million BTC were liquid, available for sale or controlled by reachable keys.

Why the milestone mattered

Block 600,000 made Bitcoin’s monetary policy observable rather than merely promised. Thousands of independently validated blocks had enforced the same height-based subsidy logic through two earlier halvings, while fees were already supplementing the miner’s subsidy in the milestone block.

The event was not itself a halving, software release or market-price record. No causal price claim is supportable from the block data, and this reconstruction makes none. Its significance was institutional and technical: a public ledger supplied a durable checkpoint for auditing issuance, while the supply caveats showed why protocol limits, issued coins and tradeable float should not be used interchangeably.

As of October 19, 2019, the next relevant monetary-policy checkpoint remained the subsidy reduction at height 630,000. Its calendar timing was uncertain because Bitcoin targets an average block interval rather than scheduling blocks by date.