Why Bitcoin Halving Calculators Are Out of Sync

The Bitcoin network halving is fast approaching, likely to fall in seven days (April 19), according to the most recent estimates. But trying to pin down the exact minute or even hour for this pre-planned, programmatic update is surprisingly unpredictable. Just take a look at any of the Bitcoin halving countdowns online: they’re all out of sync!

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Watcher Guru, for instance, shows the halving will strike in seven days, seven hours and 20 minutes, while CoinMarketCap says it will happen two hours later. The “Bitcoin Block Reward Halving Countdown” says it’ll be in seven days and 15 hours. While these estimations are all generally aligned, for someone looking to trade on the halving may be a bit frustrated.

The Bitcoin halving is scheduled to occur every 210,000 blocks, or roughly every four years. This particular event will be executed automatically by the network at exactly blockheight 840,000. Based on the way Bitcoin’s creator Satoshi Nakamoto designed the system, bitcoin miners “find” the next block to hash to the blockchain every 10 minutes, meaning it should be easy to figure out precisely when the next halving should happen down to the minute.

In practice, however, things are a bit messier.

“Calculating the time to bitcoin halving has three important elements: The current block height, the block at which the next halving occurs and the average block time,” according to Simon Cousaert, director of data at The Block Research (which currently predicts the halving will fall in seven days, 15 hours and 40 minutes).

“Since the second element, the target block, is a constant, the accuracy of the countdown depends on the current block height and the average block time,” he said. Likewise for counting the current block height, which should be easy data to fetch.

Where the discrepancy likely comes in is how different halving calculators are counting the time between blocks. Again, theoretically the time to mine a block should be even 10 minutes, by design. But the amount of miners directing computational power to mine and secure the Bitcoin blockchain is not static, meaning that that figure can fluctuate.

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“The average block time is harder to estimate accurately,” Cousaert said. “One can take a simple constant, and assume that every block takes 10 minutes to be mined.” But what halving calculators are likely doing is taking “the rolling average block time” over some time scale, whether it’s over the past 100, 90 or 30 days (or any other arbitrary period of time).

“This does not necessarily lead to a more accurate prediction, because the average of the last days does not necessarily predict the average for the next few days,” Cousaert added.

NiceHash lead mining manager Marko Tarman echoed this point saying that everyone in the world has the same access to some “static” data regarding Bitcoin: the halving block height. However, there are two other “dynamic pieces of information,” the current block height and block time.

“It's important to note that block times can fluctuate considerably,” Taman said. “If the average block times are shorter than 10 minutes, the predicted halving event will appear to be sooner. Conversely, if the average block times exceed 10 minutes, the halving event will seem to be delayed.”

In other words, counting down to the halving — an event not only known in advance, but actively anticipated to the point that people argue over whether it is “priced in” — is more of an art than a science.

“While this detail may not be critical when observing the countdown a year in advance, accuracy becomes increasingly important as the event approaches,” Tarman said.