Bitcoin halving countdown

Each block of Bitcoin pays its miner a subsidy of new bitcoins, besides the fees of the transactions in it. The halving is the moment the subsidy is cut in half. It happens every 210,000 blocks, which is about every four years at the one block every ten minutes that the protocol aims for. The subsidy started at 50 BTC and the table above lists each halving so far.

The schedule is a rule of the protocol that every node checks, so it needs no central party to carry it out, and no one can change it unless the network agrees to run different rules. It adds up to a supply that ends at 20,999,999.9769 BTC, just under 21 million: the subsidy reaches 1 satoshi, the smallest unit, and the halving after that sets it to zero, around the year 2140. From then on miners are paid by fees alone.

Traders watch the halving because it cuts the rate at which new bitcoins enter the market. How the price reacts is not something the schedule decides.

The height of the newest block is read from a public block explorer. The blocks left are exact: they are the height of the next halving block, a multiple of 210,000, minus the height of the newest block.

When the halving falls is an estimate, because blocks are found at random. The page gives two. At the recent pace, it is the time of the newest block plus the blocks left times the average time a block took over the last 2,016 blocks, the span that Bitcoin itself re-aims its difficulty over. At the 10-minute target, it is the same sum with the pace the protocol aims for. The difficulty is re-aimed every 2,016 blocks, so the recent pace stays near ten minutes, but it drifts when the hashrate rises or falls between two adjustments.

Even with a steady hashrate the date is uncertain: the time of each block is random, and the recent pace was itself measured from a limited number of blocks. The page shows the standard deviation of the date it gives, which the real date stays within roughly two times in three. A change of the hashrate moves it further. The nearer the halving, the smaller the error: in the last days it is a matter of hours.

Heights and times of blocks come from the public Esplora API of the block explorer named under the figures. This page keeps what it read for a minute. The block reward and the issuance are not read from the explorer: they are worked out from the issuance rules of Bitcoin, which halve a subsidy of 50 BTC once for each 210,000 blocks, to the satoshi.

The time of a block is stamped by its miner. The rules require it to be later than the median time of the eleven blocks before it and no more than two hours ahead of the clock of the node that checks it, so the time of a block can be off by up to a couple of hours. The times of the halvings above are those of the halving blocks themselves.

Bitcoin issued is what the rules allow to have been issued by the newest block. A miner may claim less than the full reward, and some bitcoins have been lost, so the coins that exist are a little fewer.