Subsidy Epochs & Halvings

The first halving occurred on November 28, 2012 at block 210,000 - reducing the subsidy from 50 ₿ to 25 ₿. The second halving followed on July 9, 2016 at block 420,000 (25 → 12.5 ₿), the third on May 11, 2020 at block 630,000 (12.5 → 6.25 ₿), and the fourth on April 20, 2024 at block 840,000, cutting to 3.125 ₿ per block.

Satoshi chose 210,000 blocks per epoch - at a 10-minute target block time that equals 210,000 × 10 min = 2,100,000 min ≈ 4 years. The first epoch alone issued 210,000 × 50 ₿ = 10,500,000 ₿, exactly half of the theoretical maximum. Each subsequent epoch issues half of the previous, giving a convergent geometric series: 21M × (1 − 1/2n) after n halvings.

The block reward consists of two parts: the subsidy (newly created coins, deterministic from height) and transaction fees (paid by senders, variable). Miners collect both for each block they find. As halvings progressively reduce the subsidy, fees must grow to sustain miner revenue and thus hashrate-backed security. Bitcoin's security model does not require the subsidy to last forever - it requires fees to become sufficient before the subsidy becomes negligible, which happens gradually over the next ~116 years.

After the last subsidy epoch (≈ 2139), miners will be compensated exclusively by fees. The block space scarcity enforced by the 1 MB base block weight limit is the mechanism by which fee pressure is maintained long-term.