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Bitcoin’s quantum threat has a number now: 7.4%.
New research calculates that the network must immediately dedicate at least that share of every block’s capacity to migrating vulnerable wallets to post-quantum cryptography or risk being unable to complete the upgrade before a credible quantum computer arrives. The finding transforms what has largely been a theoretical debate into a capacity-planning emergency with a ticking deadline.
The math starts with Bitcoin’s unspent transaction output set. At least 186.7M UTXOs remain secured by elliptic curve digital signature algorithm, or ECDSA-256 – the cryptographic backbone that a sufficiently powerful quantum machine could crack, exposing private keys and enabling theft at scale. Packing migration transactions into every block with no room for ordinary payments, the theoretical minimum migration window is 76 days. That is an absolute floor, not a plan.
Bitcoin has at least 186.7 million UTXOs that need migrating to post-quantum cryptography before quantum computers break ECDSA-256.
The absolute minimum time to do this from my research paper, packing every block optimally, zero normal transactions, is 76 days. That's not… pic.twitter.com/W6nxHK7HGO
— Dr. Joseph Kearney (@Joseph_Kearney) March 5, 2026
In practice, Bitcoin cannot suspend commerce during an upgrade. The network must keep processing normal transactions alongside migration, which is what produces the 7.4% figure – the minimum block-space reservation required if migration begins today and must finish before a credible quantum threat materialises.
The deadline anchoring that estimate is IonQ’s public roadmap: roughly 1,600 logical qubits by the end of 2028, a threshold many researchers regard as sufficient to threaten ECDSA-256 under optimistic assumptions. From today, that leaves approximately 1,032 days. Every day migration does not begin, the required percentage rises.
Migration has not started. There is no agreed post-quantum signature scheme, no formal Bitcoin Improvement Proposal on an upgrade path, and no community consensus on how to proceed.
Bitcoin’s last major cryptographic overhaul, SegWit, took roughly four years from proposal to activation and that fight nearly split the network. A post-quantum upgrade would require choosing among competing candidate algorithms, including lattice-based and hash-based schemes, none of which has won broad acceptance in Bitcoin development circles.
The arithmetic of delay is unforgiving. A six-month postponement meaningfully increases the daily throughput required; a full year could push the reservation figure toward territory that would visibly crowd out ordinary transactions, spike fees, and test the political tolerance of miners and users alike.
Bitcoin recently surpassed 20 million coins in circulation (over 95% of supply), out of a hard-capped supply of 21 million, a milestone underscoring both the network’s maturity and the concentration of value now sitting in potentially vulnerable addresses. At current prices, the assets exposed to a quantum attack represent trillions of dollars.
The 20 millionth bitcoin just entered into circulation via block 940,073.
Less than 1 million bitcoin remain available to be mined; it will take over a century to do so.
— Jameson Lopp (@lopp) March 10, 2026
Block space is already a bitterly contested resource. The emergence of Ordinals and BRC-20 tokens in 2023 ignited fierce debate about non-financial uses of the blockchain. A mandatory reservation of even 7.4%, a floor that rises with each day of inaction, would represent one of the most consequential and contentious reallocation decisions in Bitcoin’s history, imposed not by choice but by physics.
The research does not predict when quantum computers will break Bitcoin. It calculates how little time remains to act as if they will.
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