The Bitcoin halving explained: what it is, when it happens, and what it does not do

The halving is a few lines of code, not a date on a calendar

Every Bitcoin block contains a special first transaction, the coinbase, in which the miner pays themselves newly created coins plus the fees from the block’s other transactions. How many new coins they may create is decided by a function in Bitcoin Core called GetBlockSubsidy. It is short enough to read in full: divide the block’s height by 210,000, throw away the remainder, and shift the starting amount of 50 coins right by that many places. Each shift halves the number.

So blocks 0 through 209,999 could create 50 bitcoin each. Block 210,000 was the first that could create only 25, block 420,000 the first that could create 12.5, and so on. The comment in the code says exactly what people mean by the halving: “Subsidy is cut in half every 210,000 blocks which will occur approximately every 4 years.”

This is a consensus rule, which means every node checks it. The coinbase output total may not exceed the permitted subsidy plus the block’s transaction fees. For example, at height 210,000 a block with zero transaction fees may claim at most 25 BTC; claiming 50 BTC would make it invalid. Claiming less than the permitted amount is allowed, so 25 BTC one block before that halving would be an underclaim, not a consensus violation. Nobody schedules a halving and nobody can postpone one. It happens when the block with the right height is found, and not before. If you want to see how the block that pays the miner is built, our article on how Bitcoin mining works walks through it.

Why “about every four years” and not exactly

Bitcoin aims for one block every ten minutes on average, a figure the white paper uses in its own arithmetic, and 210,000 blocks at that pace is a little under four years. But the pace is a target, not a clock. The difficulty adjustment periodically nudges the rate back toward the target, and in between, blocks arrive faster when more mining power joins and slower when it leaves. In practice each of the four halvings so far has arrived a little sooner than four years after the one before: November 2012, July 2016, May 2020, April 2024.

Satoshi Nakamoto described the schedule in years, though, because that is how people think. In the email announcing the first release of the software on 8 January 2009, Satoshi wrote: “Total circulation will be 21,000,000 coins. It’ll be distributed to network nodes when they make blocks, with the amount cut in half every 4 years,” and listed the first steps: 10,500,000 coins in the first four years, then 5,250,000, then 2,625,000, then 1,312,500. Keep halving those amounts forever and they approach 21 million without quite reaching it.

Four halvings so far, each visible in a single block

Because the halving is tied to a block height, you can look up the exact block where each one happened. The first was block 210,000, mined on 28 November 2012, the first block whose coinbase created 25 bitcoin rather than 50. The second was block 420,000 on 9 July 2016, down to 12.5. The third was block 630,000 on 11 May 2020, down to 6.25. The fourth was block 840,000 on 20 April 2024, down to 3.125.

The fourth is worth a closer look because it shows the other half of a miner’s income. The explorer page for block 840,000 lists a total reward of about 40.75 bitcoin, of which only 3.125 was new coins; the remaining 37.6 or so came from transaction fees, as users paid heavily to get into the first block of the new era. That was unusual, but it illustrates the design: as new-coin rewards shrink, fees are meant to carry more of the load.

Our dossier on Bitcoin halvings and supply records the header timestamp of each halving block to the second, works through the exact arithmetic of the cap, and explains why the true maximum is a hair under 21 million. This article keeps to the plain version.

Why Satoshi built the supply this way

The white paper gives the reason in one paragraph. New coins are how the network pays the people who secure it, and how coins get into circulation at all: creating them with each block “adds an incentive for nodes to support the network, and provides a way to initially distribute coins into circulation, since there is no central authority to issue them.” Satoshi compared it to “gold miners expending resources to add gold to circulation,” except that here the resource is “CPU time and electricity.”

The same paragraph explains why the reward does not last forever. “Once a predetermined number of coins have entered circulation, the incentive can transition entirely to transaction fees and be completely inflation free.” The halving is the mechanism for that handover: generous at first, when the network needed to bootstrap and distribute coins, and tapering toward zero as fees take over. The 21 million figure Satoshi gave in the January 2009 email is simply where that taper ends up.

The first block ever mined created 50 coins, and you can inspect that first coinbase in our genesis block dossier. Every reward since has followed the same function, which is why anyone can count Bitcoin’s supply from first principles. Our article on how many bitcoins there are does that count.

Halving the subsidy does not halve all miner revenue

The subsidy is newly issued bitcoin permitted by the block-height rule. Transaction fees are a separate component of the miner’s allowed reward. Halving the subsidy does not mechanically halve fees or total revenue.

For an illustrative block with a 3.125 BTC subsidy and 0.20 BTC of fees, the combined amount is 3.325 BTC. These are example fee inputs, not an observation of a particular block. The issuance rule also says nothing about a guaranteed market-price response.

When the last halving happens

Bitcoin’s smallest unit is the satoshi, and the code defines one bitcoin as 100,000,000 of them. The subsidy is calculated in satoshis as a whole number. Fifty coins is five billion satoshis. Halve that 32 times and you are left with a single satoshi; halve it once more and the whole-number arithmetic rounds to zero. So the 33rd halving, at block 6,930,000, is the one that ends new issuance. At ten minutes a block, that is around the year 2140.

The code also has a guard that forces the reward to zero after 64 halvings, but that is a safety measure against a programming edge case, not the moment issuance stops; the number was already zero long before. From then on, miners are paid entirely in fees, exactly as the white paper anticipated. The next halving on the way is block 1,050,000.