Segwit2x: What you need to know about the 2x Hard Fork (aka 2MB non-Segwit Transaction Capacity per block/8MB Total Block Size Hard Fork)

Yet another BIP — BIP102

BIP102 is a flag-day hard fork to 2MB, written in 2015 by Jeff Garzik. Jeff Garzik created a pull request to the Bitcoin Core repository on July 16, 2015 and after 8 months of discussion, it was never merged. Jeff Garzik closed this pull request in September of 2016 as it seemed pretty clear that BIP102 was not getting merged into Core.

Segwit2x Contents

It may be useful at this point to look at what Segwit2x actually is.

Who’s going to run this software?

The big question, then, is who’s going to be running Segwit2x? Who is willing to run a Bitcoin client that’s not Bitcoin Core? Well, according to the New York Agreement, everyone who’s signed the agreement has essentially agreed to run this software. Based on the latest statistics from, that’s roughly 87% of mining hash power, a lot of exchanges, a lot of merchants and a lot of wallets. If these companies do what they said they’d do in the agreement, that would represent a significant portion of the economic nodes in Bitcoin.

What can we expect?

The easiest way to think about what will happen is with some rough dates:

  • July 21 — Segwit2x is due to be released and presumably all participants to the New York Agreement will run it. The miners currently signaling “NYA” in their coinbase transactions, or about 87% of the last 24 hours’ worth of blocks, are a good estimate of blocks that will mine using Segwit2x.
  • July 23 — BIP91 should lock in (requires 80% of 336 blocks to signal)
  • July 26 — BIP91 should activate. At this point, all blocks are required to signal for Segwit (BIP141) or they will be orphaned off the network.
  • Around July 26–27 — This is also around when we can expect a new difficulty adjustment period to begin. The last one as of this writing was June 17 and they typically take 12–14 days (closer to 13 since new hardware comes online constantly). So June 30 (block 473760), July 13 (block 475776) and July 26 (block 477792) seem to be the next 3 difficulty adjustment period starts.
  • August 10 — This is around when we can expect block 479808, which is the end of the difficulty adjustment period. Very close to 100% of blocks in this difficulty adjustment period should be signaling for BIP141 (Segwit). As this is above 95%, Segwit should be locked in.
  • August 23 — This is around when we can expect block 481824, which is when Segwit should activate. From this block on, Segwit transactions will be legal on the network. This also sets the activation date for BIP102 on Segwit2x to block 494784 (exactly 144*90 blocks later).
  • November 18 — This is around when we can expect block 494784, which is when Segwit2x should hard fork to 2MB blocks.

What this means for you

Practically speaking, as long as Segwit2x releases something that works for mining by July 21, we should get Segwit on the network around August 23. That means that Lightning Network transactions should be available soon after and everyone will be watching to see if block congestion and transaction fees reduce.


Segwit looks like a reality on the network, but with it comes a 2MB hard fork. Whether the 2MB hard fork will have teeth is an open question, but the fact that a significant percentage of the economic nodes have signed an agreement to hard fork means, well, that there will probably be a hard fork. If there is even a little resistance to a hard fork, we can expect two Bitcoins around November 18.



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Jimmy Song

Jimmy Song


Bitcoin Educator, Developer and Entrepreneur. Book: PGP Fingerprint: C1D7 97BE 7D10 5291 228C D70C FAA6 17E3 2679 E455