Bitcoin Difficulty Fell 16.05% in Its Biggest Cut Since 2011

Bitcoin’s mining difficulty fell 16.05% at block 655,200 on November 3, 2020, the network’s largest percentage reduction since October 2011 and the second-largest downward adjustment then recorded.

The block’s recorded timestamp was November 3 at 08:28:10 UTC. Difficulty moved from approximately 19.997 trillion to 16.788 trillion, a 16.0499% decline calculated from the two reported difficulty levels and rounded to 16.05%. Difficulty is a dimensionless protocol measure, not a price, hash-rate reading or quantity of computing equipment.

The event mattered because it demonstrated Bitcoin’s automatic response to a substantial contraction in the computing power competing to produce blocks. Rather than requiring an administrator or emergency software release, the protocol reduced the proof-of-work threshold after the preceding 2,016-block period took longer than its intended two-week schedule.

How the retarget worked

Bitcoin Core’s contemporaneous rules targeted one block approximately every ten minutes and recalculated the proof-of-work requirement after each 2,016-block interval. When blocks in the completed interval arrived too slowly, the next interval became easier; when they arrived too quickly, it became harder. The permitted adjustment was bounded, limiting how much the target could change in one retarget.

Block 655,200 began the new interval. Its lower difficulty did not restore disconnected machines, add energy to the network or guarantee ten-minute blocks. It changed the amount of expected computation required to find a valid block, allowing the remaining mining population to produce blocks closer to the intended cadence if total hash power stayed broadly stable.

The 16.05% cut also did not mean that Bitcoin’s security had fallen by exactly 16.05%. Difficulty and estimated hash rate are related but distinct measurements. Hash rate cannot be observed directly across the entire network; it is inferred from difficulty and the random timing of discovered blocks. Short measurement windows can therefore produce noisy estimates.

A seasonal mining disruption

Contemporaneous industry reporting attributed much of the preceding slowdown to miners in China’s Sichuan province switching off or relocating equipment as the rainy season ended and abundant hydroelectric power became less available. That explanation was plausible within the mining industry’s known seasonal operating pattern, but it remained an attribution based on reporting and operator accounts—not a complete census of every machine that disconnected.

Other causes could have contributed to the observed block interval, including ordinary statistical variance, equipment economics and operational decisions elsewhere. The blockchain establishes that blocks arrived slowly enough to trigger the retarget; it does not encode why individual miners stopped hashing.

For miners that remained online, the change improved expected economics relative to the preceding difficulty period. Holding bitcoin’s price, transaction fees, equipment efficiency and electricity costs constant, the same amount of hash power represented a larger share of the competition for each block subsidy. That is a mechanical interpretation, not evidence that every miner became profitable or that realized revenue rose by precisely 16.05%.

Why November 3 mattered

The adjustment arrived while bitcoin’s market price was advancing and mining remained geographically concentrated enough for a regional power transition to affect global block production. It exposed that concentration while also displaying the protocol mechanism designed to accommodate changing participation.

The defensible event-day conclusion was therefore narrower than either a security alarm or a celebration of mining profits: Bitcoin experienced an unusually large loss of effective mining power during one adjustment window, then automatically lowered difficulty at a recorded block boundary. Whether relocated machines would return, how quickly block timing would normalize and how long improved miner margins would last remained uncertain on November 3, 2020.