Bitcoin Halving Schedule and the 21 Million Supply Cap

Each block that a miner finds starts with a coinbase transaction. It pays the miner the block subsidy plus the transaction fees. The subsidy is the only way that new bitcoin are created. The Bitcoin white paper (Satoshi Nakamoto, 2008, on bitcoin.org) calls it a special transaction that starts a new coin owned by the creator of the block. When the network started on 3 January 2009, the subsidy was 50 BTC per block.

The schedule is a consensus rule in the node software. In Bitcoin Core, the function GetBlockSubsidy (src/validation.cpp) divides the block height by 210,000 to get the number of halvings, returns zero when that number is 64 or more, and otherwise shifts 50 BTC (5,000,000,000 satoshis) right by the number of halvings. The 210,000 interval is the nSubsidyHalvingInterval parameter in src/kernel/chainparams.cpp.

The difficulty adjustment targets one block every 10 minutes, so 210,000 blocks take about four years. A halving happens at a fixed block height, not on a fixed date.

If the subsidy halved exactly forever, the total would approach 50 × 210,000 × 2 = 21,000,000 BTC without reaching it. But Bitcoin counts in satoshis (1 BTC = 100,000,000 satoshis), and the right shift in GetBlockSubsidy drops any fraction of a satoshi.

The first nine halvings divide evenly. At halving 10 (block 2,100,000) the exact value would be 4,882,812.5 satoshis, so each block pays 4,882,812, and every later era also rounds down. In era 32 the subsidy is 1 satoshi, and at halving 33 (block 6,930,000) the shift reaches zero. Add up all 33 eras and the result is 2,099,999,997,690,000 satoshis, which is 20,999,999.9769 BTC.

The constant MAX_MONEY of 21,000,000 BTC in src/consensus/amount.h is only a sanity limit for amounts, not the issuance schedule.

These totals follow from the subsidy rule. The coins that can actually move will be fewer: the 50 BTC in the genesis block cannot be spent, some miners claimed less than the full subsidy, and coins on lost keys stay where they are. Nobody can measure lost coins exactly, so this page does not estimate a number.

A halving cuts the new supply per block in half at a fixed height. Miner income is the subsidy plus fees, so each halving makes fees a larger share of it. The white paper expected this: once a predetermined number of coins have entered circulation, the incentive can transition entirely to transaction fees.