The Bitcoin community skilled a uncommon two-block reorg on Mar. 23, at block top 941,880. Foundry mined six consecutive blocks, AntPool and ViaBTC briefly prolonged a competing department.
The chain resolved the fork as designed, following the trail with probably the most hash charge. Bitcoin carried out precisely as designed and validated its assumptions.
The heuristic no one labeled
The six-confirmation rule is likely one of the items of acquired knowledge which have traveled so removed from their origins that most individuals who repeat it will possibly’t reconstruct why six is the quantity.
The reply traces again to Satoshi Nakamoto’s 2008 whitepaper, which modeled finality as a catch-up likelihood. As sufficient blocks pile up on high of a transaction, the computational value of rewriting historical past turns into prohibitive for an attacker with restricted hashpower.
Six blocks grew to become the neighborhood shorthand for “protected sufficient,” despite the fact that the whitepaper handled it as a calculation that assumes the attacker controls about 10% of the community’s hashpower.
That assumption has been quietly doing plenty of work for sixteen years.
Jameson Lopp made the implication specific in an evaluation of affirmation threat. The consolation stage baked into six confirmations is a perform of who else is on the community and the way a lot of it they run.
Beneath the Nakamoto catch-up mannequin, six confirmations in opposition to an attacker holding 10% of hashpower yields a reversal threat of roughly 0.02%. Towards 20%, that determine climbs to about 1.43%. Towards 30%, it reaches roughly 13.2%.
On the 32.2% share Foundry held in latest pool-share snapshots, the identical mannequin places six-confirmation reversal threat close to 18.9%.
Mining swimming pools will not be coordinated attackers by default, which is why they do not slot in these mannequin outputs. Foundry USA describes itself as an institutional-grade pool constructed for miners that coordinates many impartial operators.
Miners can and do swap swimming pools, making an overt assault can be economically self-destructive for any rational pool operator. Focus in block manufacturing modifications the danger mannequin individuals use to determine when a cost feels last, no matter how dispersed the underlying machines are.
A 2022 latency safety evaluation famous that with a ten% adversary and a 10-second propagation delay, six confirmations nonetheless produce a safety-violation likelihood between 0.11% and 0.35%.
Six was by no means a tough ceiling, even below circumstances way more favorable than these of at present.
Three circumstances without delay
The context surrounding the reorg carries the load.
Bitcoin’s community is at present working three circumstances concurrently that put the six-confirmation heuristic below strain, which it has not often confronted in apply.
Up to now three days, Foundry has held roughly 31% of the worldwide hashrate, whereas AntPool sits at about 18.4%, and ViaBTC at 10.5%, in keeping with Hashrate Index information. These three swimming pools mixed account for roughly 60% of block manufacturing.
That diploma of focus in coordinator energy is elevated by any affordable measure during the last a number of years.
On the identical time, mining economics have deteriorated sharply. Problem dropped 7.76% on Mar. 21 in one in every of 2026’s largest unfavourable changes. Hashprice averaged $32.31 per petahash per day in February, down almost 18% month over month, briefly touching a document low of $27.89.
Transaction charges contributed simply 0.57% of whole block rewards within the final 24 hours of accessible information.
When margins compress and payment income dries up, smaller and mid-sized miners face a rising incentive to pool into whichever coordinator provides the most effective variance discount. This often means the already-large swimming pools get bigger.
The January winter storm provided a counterpoint price noting. Foundry’s hashrate reportedly dropped by round 60%, or almost 200 exahashes per second, throughout that interval, demonstrating that pool shares can redistribute shortly when exterior circumstances change.
Amid this backdrop, the six-confirmation rule lacks an automated adjustment mechanism when pool shares transfer.
In apply, the trade’s largest venues have deserted the six-confirmation commonplace in a quiet operational judgment made years in the past.
Coinbase requires two confirmations for $BTC deposits to be marked as pending, whereas Kraken and Gemini every require three.
None of these thresholds is unsuitable for his or her use circumstances: for atypical retail deposits, two or three confirmations characterize a completely defensible threat tolerance.
The hole between these real-world numbers and the people commonplace of six illustrates that “six confirmations” was all the time extra a cultural artifact than a common coverage.
Lopp’s framework argues that this hole ought to develop extra deliberate. Required confirmations ought to scale with transaction worth and the economics of the attacker.
A $500 retail deposit and a $50 million OTC settlement don’t share the identical threat profile, and the sincere model of finality steerage would explicitly state so.
The quantity that stayed the identical
There are totally different outcomes within the present hashrate focus situation, which raised an alarm for customers.
Positively, hashrate redistributes throughout a broader pool of coordinators as mining margins ultimately recuperate and new entrants compete for share.
The January storm already demonstrated that Foundry’s dominance can erode shortly below the appropriate circumstances. If focus eases and the hash worth recovers, six confirmations stay an affordable default for big $BTC settlements.
On the flip facet, Foundry might stay above 30%, and the top-three focus stays sticky. No malicious occasion is required for the norm to degrade, as exchanges, OTC desks, and retailers dealing with high-value transfers can quietly elevate inner thresholds or formalize dynamic tiers tied to observable pool-share information.
Beneath the Nakamoto mannequin, six confirmations in opposition to a totally coordinated 32.2% attacker leaves roughly 18.9% catch-up threat, a determine genuinely tough to reconcile with language like “successfully irreversible” for transfers within the tens of thousands and thousands of {dollars}.
The state of affairs requires solely that the pool focus stay the place it’s, whereas the hole between the people commonplace and the precise threat widens sufficient that somebody with cash on the road stops ignoring it.
Bitcoin’s settlement assurances have been all the time “six blocks, below a sure distribution of hashpower and a sure tolerance for threat.”
The 2-block reorg produced a uncommon second when the hole between Bitcoin’s finality folklore and its underlying math grew to become arduous to disregard.
Contemplating this second, the six-confirmation rule’s days as a common, unqualified commonplace are working out.
