What a halving is
Every Bitcoin block pays its miner a subsidy of newly issued coins. Every 210,000 blocks that subsidy is cut in half. It has happened four times so far, taking the reward from 50 BTC to 3.125 BTC.
Block reward reduction
Approximately every 210,000 blocks, the Bitcoin block subsidy is cut in half to control issuance and ensure scarcity.
Every Bitcoin block pays its miner a subsidy of newly issued coins. Every 210,000 blocks that subsidy is cut in half. It has happened four times so far, taking the reward from 50 BTC to 3.125 BTC.
The schedule is written into the consensus rules and is the whole of Bitcoin's monetary policy: issuance falls on a fixed curve towards a supply cap of 21 million, with no discretion and nobody to petition.
The countdown above multiplies the blocks remaining by 10 minutes, the protocol's difficulty target. Real block times vary with hash rate: faster blocks pull the date earlier, slower ones push it later, and difficulty only re-adjusts every 2,016 blocks. Expect the date to move.
A halving is triggered by a block height, not a calendar date. Block 1,050,000 will halve the subsidy whenever it is mined, which is why the block figures on this page are exact and the date is not.
Block height from mempool.space. Dates are estimates and subject to change.