What Happens During a 51% Attack?

What would it actually take to cheat Bitcoin, and why hasn’t anyone successfully done it at scale?

A 51% attack refers to a scenario where a single miner or coordinated group gains control of more than half of Bitcoin’s total network hash rate. Because the longest-chain rule means the network always follows whichever chain has the most accumulated Proof of Work, controlling a majority of hash rate would let an attacker consistently out-mine everyone else combined.

What could an attacker actually do with that majority control? The most commonly cited risk is double-spending: an attacker could send a transaction, wait for it to be confirmed and used to receive goods or services in return, then secretly mine an alternative chain that doesn’t include that transaction, and release it once it’s longer than the public chain.

What a 51% attack cannot do is just as important. Even with majority hash rate, an attacker cannot create new coins out of thin air, cannot steal coins from wallets they don’t control the private keys to, and cannot change Bitcoin’s core rules like the 21 million coin supply cap. Every node still independently checks the basic rules of the protocol.

Why hasn’t this happened to Bitcoin? Simple economics. With hundreds of exahashes per second of honest hash rate securing the network today, acquiring majority control would require an almost unimaginable amount of specialized hardware and electricity, all while an attack that becomes public would likely crash Bitcoin’s price and destroy the attacker’s own investment.

Smaller cryptocurrencies with far lower hash rates than Bitcoin have suffered real 51% attacks in the past — a reminder this isn’t purely theoretical in the broader crypto space, but Bitcoin’s enormous, globally distributed hash rate makes the attack economically irrational here.

Want to understand exactly how hash rate gets measured and tracked? Continue learning in the Bitcoin Academy.