The block size war

Wonder

1. Why did a fixed 1 MB block size, added quietly in 2010, become the central governance crisis of 2017?
2. Why couldn’t developers, miners and businesses simply agree on a number?
3. How can a community with no formal authority agree on protocol changes without a vote or a leader?

Explore

1. Satoshi Nakamoto added a 1 MB block size limit to Bitcoin in 2010, widely believed to be an anti-spam measure; he never explained it publicly. A block held a few thousand typical transactions, ample at first but far short of global scale. As use grew through 2015 and 2016, blocks filled, waits lengthened, and by late 2017 average fees reached tens of dollars.
2. The ‘Big Blockers’ camp argued Bitcoin must grow its on-chain capacity to function as a payment system. Gavin Andresen, former lead maintainer of Bitcoin Core after Satoshi’s departure, wrote BIP101 in 2015, proposing to raise the limit from 1 MB to 8 MB in 2016 and double it every two years to about 8 GB by 2036; Mike Hearn co-led Bitcoin XT, which adopted it. Roger Ver and, later, Bitmain’s Jihan Wu were prominent supporters of bigger blocks. Bitcoin XT, an alternative client, adopted BIP101 in August 2015, but it needed 75% of miners to activate and never came close.
3. The opposing camp argued that larger blocks would centralize validation. A full node must download, store and verify every block; at 8 MB or more, hardware costs rise sharply, pushing average users out of running nodes. Consensus would then rest with a few well-resourced entities—miners and exchanges with data center infrastructure—undermining Bitcoin’s decentralized security model. This camp favored efficiency gains within the existing structure and layered scaling solutions like the Lightning Network.
4. Attempts at consensus failed. The Scaling Bitcoin conferences in 2015 produced no agreement. The Hong Kong agreement of February 2016, brokered between developers and miners, proposed a path to larger blocks but collapsed as implementation details diverged. Mike Hearn left in January 2016 with an essay declaring that the bitcoin experiment had failed, and Bitcoin Classic and Bitcoin Unlimited took up the big-block cause.
5. The decisive events came in August 2017. On 1 August, a group of miners and developers launched Bitcoin Cash by hard fork, with an 8 MB limit. Segregated Witness, a soft fork already locked in by miners, activated on 24 August: it replaced the 1 MB size limit with a 4 million weight-unit limit, raising effective capacity while old nodes still saw blocks of at most 1 MB of non-witness data. The chain split permanently: Bitcoin Core followed the soft fork path; Bitcoin Cash pursued larger blocks.
6. How to read the outcome is still disputed. The small-block view holds that users and economic nodes, choosing which software to run, decided which chain kept its value; the big-block view holds that miners and businesses chose to activate SegWit through their own agreements, and that Bitcoin Cash preserved the original vision. Miners signal through version bits; exchanges, wallets and merchants decide which coins they accept. The SegWit2x hard fork, proposed as a follow-up to raise Bitcoin’s limit to 2 MB, was called off in November 2017 as support among businesses and node operators collapsed. Most histories treat that cancellation as the end of the war.

Key facts

Exercise nine, six, three, one

Fact check

Dates, figures and claims checked against the published sources listed below.

• J. Bier, The Blocksize War (2021); R. Ver & S. Patterson, Hijacking Bitcoin (2024), for the big-block perspective
• BIP101: Increase maximum block size (Gavin Andresen, 2015)
• M. Hearn, 'The resolution of the Bitcoin experiment' (January 2016)
• BIP141: Segregated Witness (Consensus layer) (Lombrozo, Lau & Wuille, 2015)
• Scaling Bitcoin Conference proceedings (2015)