The short version
A halving is a scheduled event in which the amount of new bitcoin created per block is cut in half. It is built into Bitcoin’s rules. No one decides on it at the time, no vote takes place, and it is not triggered by price or by any person. It happens automatically when the chain reaches a particular block height.
The block reward schedule
Bitcoin’s blocks are added about once every ten minutes on average. The reward each block pays to its miner, the block subsidy, started at 50 bitcoin. The rule is that the subsidy halves every 210,000 blocks.
| Block height where subsidy changes | New subsidy |
|---|---|
| 0 (start, 2009) | 50 |
| 210,000 | 25 |
| 420,000 | 12.5 |
| 630,000 | 6.25 |
| 840,000 | 3.125 |
| 1,050,000 | 1.5625 |
The first four halvings took place in November 2012, July 2016, May 2020 and April 2024. At a perfect ten minutes per block, 210,000 blocks take 2,100,000 minutes, which is a little under 4 years. That arithmetic is the source of the “every four years” shorthand.
What happens technically
Each block begins with a special transaction, called the coinbase transaction, in which the miner creates the reward and sends it to their chosen address. Every node on the network checks, when it receives a block, that the amount claimed is no more than the subsidy allowed at that height plus the fees in the block. A block that claims too much is rejected, no matter how much work was spent on it.
The code calculates the allowed subsidy from the block height. It counts how many times 210,000 blocks have passed and shifts the starting value of 50 bitcoin, measured in satoshis, to the right by that number of steps. A right shift halves a whole number and drops any fraction. This is why the real total supply is slightly under 21 million, and why, after 33 halvings, the subsidy rounds to zero. Bitcoin’s units are explained in the article on satoshis.
So at height 839,999 a miner may claim 6.25 plus fees, and at height 840,000 only 3.125 plus fees. Nothing else happens at that moment: no software switch, no pause.
Why the date drifts
The halving is tied to block height, not to a date. If blocks are found faster than ten minutes on average, the halving arrives sooner than four years; if slower, later. In practice, the network adjusts the mining target every 2,016 blocks, roughly every two weeks, to keep the average close to ten minutes, a mechanism covered in the article on difficulty adjustment. Hash rate has grown over time, which tends to speed up blocks until the next adjustment, so halvings have tended to arrive a little earlier than an exact four-year schedule. Estimated dates published ahead of time are projections.
What changes and what does not
What changes: the rate of new coin creation drops by half. Miners’ subsidy income in coins falls by half unless fees rise, so miners with higher costs may find operating less profitable. Their revenue in dollar terms depends on the price at the time.
What does not change:
- The maximum of 21 million coins. The halving is the mechanism that produces the cap.
- The target of about ten minutes per block.
- The rules for transactions, wallets and fees. Existing coins are not affected.
- Block size or the speed at which transactions are confirmed.
The halving does not remove coins from anyone’s wallet, and it does not create a sudden shortage. It slows new supply, which in relative terms is already small, as the article on the issuance rate explains.
Effects on miners
When the subsidy halves, a miner earning the same fees and facing the same costs earns less. Some respond by upgrading to more efficient machines or finding cheaper electricity; those who cannot may shut down, at least until the difficulty adjusts to the lower hash rate. Over the long run, transaction fees are expected to matter more, a topic covered separately. The general topic of how bitcoin mining works from hashing to block rewards is also its own article.
Effects on price
The halving is sometimes described as a reason for prices to rise, because supply growth slows. The reality is less clear. The date is known years in advance, so traders can already take it into account, and prices are driven by many other things. This article does not predict any price response, and the reasons for caution are discussed under halving myths. A factual account of what happened around earlier halvings is in the halving history article.
Takeaway
The halving is a fixed rule that cuts the new-coin reward by half every 210,000 blocks until the reward reaches zero around the year 2140. Understanding it helps in reading news and data, but it does not tell you where prices are headed. Cryptocurrency prices are volatile, past performance does not predict future results, and you could lose everything you invest. This is educational content and is not financial, tax or legal advice.