How much would it cost to 51% attack Monero?

There is no honest fixed dollar price for a 51% attack on Monero. The cost depends on how much computing power honest miners currently supply, how long an attacker wants control, and whether enough suitable hardware can actually be obtained.

What “51%” means for the cost

A 51% attack means controlling a majority of the computing power mining a blockchain—the shared record of transactions. That computing power is called hashrate.

The important detail is that an attacker adding new machines would need to exceed the existing honest hashrate, not buy power equal to 51% of it.

For example, if honest miners supplied 100 units of computing power, an attacker adding 51 units would control only about 34% of the combined total. They would need more than 100 additional units to hold a majority.

If existing miners were redirected instead, the calculation would differ because that power would leave the honest side. This is one reason a simple “network hashrate × rental price” estimate can mislead you.

What you would need to price

A useful estimate separates several costs:

• Hardware: Buying or securing enough suitable processors, plus memory, motherboards, and supporting equipment.
• Electricity: Running those systems, including cooling and power-supply losses.
• Hosting and operations: Space, maintenance, connectivity, and people to manage the equipment.
• Rental charges: If computing power is rented, the actual price and availability at that scale.
• Duration: Maintaining control costs more over hours or days than over a short period.

Buying hardware creates a large upfront cash requirement, but it is not necessarily all a permanent loss: machines might retain resale value. Rental spending is generally spent outright.

Mining rewards could offset some operating costs if the attacker’s blocks remain accepted. But you should not treat those rewards as guaranteed, or subtract them using an assumed future XMR price.

Why Monero is difficult to price this way

Monero uses RandomX, its mining algorithm since November 30, 2019. An algorithm is the set of rules computers follow to perform mining work. RandomX favors general-purpose processors, or CPUs.

That makes ordinary server hardware relevant, but it does not mean a majority of Monero’s hashrate is sitting available for rent.

RandomX also needs substantial memory resources. Its fast mode uses a dataset of about 2,080 MB, and efficient mining generally needs about 2 MB of L3 cache—fast memory inside the processor—per mining thread.

Real performance therefore varies between machines. Advertised CPU counts alone cannot tell you the cost. A credible estimate needs current network data, measured RandomX performance, local electricity prices, and genuine capacity quotes. A live explorer and mining calculator provide starting inputs, not a complete attack price.

Cost does not guarantee success

Majority control could let an attacker censor transactions or reverse some recent payments, potentially enabling a double-spend: paying someone, then replacing the payment history.

It would not let them steal arbitrary wallets, forge signatures, or create coins outside Monero’s rules.

A narrow majority also does not guarantee a successful reversal within a chosen time. Mining involves chance, recipients may require more confirmations, and exchanges could suspend deposits. Those responses affect whether the attacker can profit.

The bottom line

You cannot reliably put one dollar figure on attacking Monero. The meaningful estimate is the cost of obtaining enough real RandomX capacity to outweigh honest miners, sustaining it, and bearing substantial uncertainty—not a neat hourly number from a calculator.