<p>When performing a <a href="https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power">51% attack</a>, you will need to find new blocks faster than the remainder of the network combined. This way, your block chain will become the longest and clients will consider it the main chain.</p>

<p>Theoretically, this is achieved (on long term) when you have >50% of the hash power.</p>

<p>But as we know, a lot of luck is involved in mining. It's possible that someone with 55% or even 60% of the total hash power will only succeed in his 51% attack after many weeks because he has bad luck. (Note that with >50%, statistically you will always succeed on long term.) But it is also possible that someone with less than 50% can succeed on short term.</p>

CC BY-SA 3.0
2013-05-15T12:11:28.820Z
In most contexts referring to majority rule, 50% + 1 of votes is generally the right way to describe the majority. 51% is shorter and less confusing though.
Stephen Gornick
CC BY-SA 3.0
2013-05-15T08:12:14.073Z
2013-05-15T08:12:14.073Z
Steven Roose
<p>Is it 51% attack or 50.0000...1% attack?</p>

<p>Bitcoin is getting to a point where 1% is a lot of hashing power...</p>

<p>My understanding is >50% due to the nature of the protocol.</p>

CC BY-SA 3.0
2013-05-15T07:02:16.043Z
Actually it's possible attempt an attack with less. You could be successful in punctual double-spends with less than 45% of the total hashing power, but it requires a bit (if not a lot) of luck.
Stéphane Gimenez
CC BY-SA 3.0
2013-05-15T07:11:55.237Z
50-51% is the point where it becomes essentially trivial to have continued [control](http://bitcoin.stackexchange.com/questions/658/what-can-an-attacker-with-51-of-hash-power-do) over block creation.
Stéphane Gimenez
CC BY-SA 3.0
2013-05-15T07:31:29.323Z
this actually makes a lot of sense. you should post this as an answer!
nehz
<security><majority-attack>
Is it a 51% attack or a >50% attack?
nehz
# Is it a 51% attack or a >50% attack?

(Asked by: username_0 on 2013-05-15)

Is it 51% attack or 50.0000...1% attack?

Bitcoin is getting to a point where 1% is a lot of hashing power...

My understanding is >50% due to the nature of the protocol.

**Tags:** security, majority-attack

> **Comments:**
> **username_1**: Actually it's possible attempt an attack with less. You could be successful in punctual double-spends with less than 45% of the total hashing power, but it requires a bit (if not a lot) of luck.  
> **username_1**: 50-51% is the point where it becomes essentially trivial to have continued [control](http://bitcoin.stackexchange.com/questions/658/what-can-an-attacker-with-51-of-hash-power-do) over block creation.  
> **username_0**: this actually makes a lot of sense. you should post this as an answer!  

---

## 1 answers

### Answer by username_2 on 2013-05-15. Score: 8 | Accepted answer

When performing a [51% attack](https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power), you will need to find new blocks faster than the remainder of the network combined. This way, your block chain will become the longest and clients will consider it the main chain.

Theoretically, this is achieved (on long term) when you have >50% of the hash power.

But as we know, a lot of luck is involved in mining. It's possible that someone with 55% or even 60% of the total hash power will only succeed in his 51% attack after many weeks because he has bad luck. (Note that with >50%, statistically you will always succeed on long term.) But it is also possible that someone with less than 50% can succeed on short term.

> **Comments:**
> **username_3**: In most contexts referring to majority rule, 50% + 1 of votes is generally the right way to describe the majority. 51% is shorter and less confusing though.  

---

