How much hashrate does an attacker need for a 51% attack on Monero?
An attacker needs more than half of the total active Monero mining power to carry out a sustained majority, or “51%,” attack. If they add new mining power while honest miners keep working, they need more power than all those honest miners combined, not just 51% of the hashrate you see before the attack.
The calculation that matters
Hashrate is the speed at which mining hardware performs the calculations used to compete for blocks. A block is a batch of transactions added to Monero’s shared record, called the blockchain.
Let H be the hashrate of miners who remain honest, and A be the attacker’s hashrate. The attacker’s share is:
Attacker’s share = A ÷ (H + A)
To exceed 50%, the attacker needs A > H. To reach exactly 51% of the combined total, they need:
A = (0.51 ÷ 0.49) × H ≈ 1.041 × H
For example, if honest miners had a hypothetical 1 GH/s—one billion hashes per second—the attacker would need slightly more than 1 GH/s for a majority, or about 1.041 GH/s for a 51% share. This is an example, not Monero’s current hashrate.
Added power is different from redirected power
The amount of new hardware needed depends on where the attacker’s power comes from.
- Entirely new power: If all existing miners stay honest, the attacker must add more than the entire existing network’s hashrate.
- Redirected existing power: If the attacker takes control of mining power already on the network, controlling more than half of that existing total can give them a majority without adding hardware.
- A mixture: Count the attacker’s controlled power on one side and the remaining honest power on the other. The attacker needs the larger share.
This distinction matters when you read claims about a mining pool becoming too large. A pool coordinates participating miners, but those miners can leave. Its advertised hashrate is not necessarily power an attacker could reliably control.
Why 51% is not a magic switch
Monero follows the valid chain with the greatest cumulative difficulty, meaning the most accumulated mining work—not simply whichever chain has more blocks.
A sustained majority lets an attacker build accumulated work faster than honest miners on average. That can let them replace recent transaction history, attempt to spend their own coins twice, or interfere with transaction confirmation.
But mining is random. A majority does not guarantee an immediate win, and an attacker with less than half can sometimes reverse a short stretch of recent history through luck. The chance depends on their share and how many confirmations—the blocks supporting a transaction—you wait for.
Even a majority cannot forge your signature, spend your coins without your keys, or make honest software accept invalid coin creation.
Getting a meaningful current estimate
There is no permanent hashrate number for attacking Monero. Check a live Monero explorer for estimated network hashrate, preferably across several time windows.
That estimate is inferred from mining difficulty and block timing; it is not a direct inventory of mining machines. Monero targets roughly two-minute blocks and adjusts difficulty every block, but those adjustments do not remove an attacker’s majority advantage.
The bottom line
For newly added attack power, use more than the existing honest network’s hashrate as the majority threshold. For exactly 51% of the resulting total, use about 1.041 times honest hashrate. Control, duration, and random block timing still determine what that power can accomplish.
