Bitcoin Cash Difficulty Adjustments

Bitcoin Cash has had a very interesting launch with miners coming in and out very quickly. In this post, I’m going to explain how the difficulty on the Bitcoin Cash (BCH) chain will change over the next 10+ blocks.

How Bitcoin Cash Retargets

Bitcoin Cash has a very particular rule about when difficulty can adjust downward. Rules for this are as follows:

  • Median Time Past of the current block and the Median Time Past of 6 blocks before has to be greater than 12 hours.
  • If so, it gets 20% easier to create proof of work. In other words, miners can find blocks 20% easier.

Median Time Past (MTP) is just the median of the last 11 blocks. That is, put the last 11 blocks in time order and pick the middle one. This is used because sometimes blocks mined later have earlier time stamps (some technical reasons related to different clocks).

The thing about MTP is that we can calculate it before a block is found. Because we’re looking at the timestamps of the last 11 blocks and picking the middle one, we don’t need to know exactly what the timestamps of the future blocks are going to be, just the earliest 6. Most of the time MTP is just the block timestamp of 6 blocks ago.


Here’s a chart to help:


There are a bunch of blocks that have not been mined yet, but even if they were instantly mined, we would have 6 difficulty adjustments on blocks 478577 to 478582. This will reduce difficulty 20% each so we will have blocks that are 0.8⁶ ~ 0.262 as hard to mine. That is, miners will need to put in about 1/4 the work to mine a block.

This may explain why there was a 13 hour gap between blocks 478570 and 478571. Miners may simply have wanted the chain to be easier to mine!

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