<p>The lesson of the recent v0.7/v0.8 fork issue is that simply waiting for 6 or more confirmations is insufficient.  You also need to satisfy yourself that there are no competing blockchain forks</p>

<p>e.g. this thread discusses a double spend proof-of-concept that was executed during the chain fork: <a href="https://bitcointalk.org/index.php?topic=152348.0" rel="nofollow">https://bitcointalk.org/index.php?topic=152348.0</a></p>

CC BY-SA 3.0
2013-03-13T13:07:27.283Z
2013-03-13T13:07:27.283Z
Richard G Brown
<p>With 6 confirmations you are safe from double spends and the like.</p>

<p>For low-value transactions, 1 or 2 are likely enough.</p>

CC BY-SA 3.0
2013-03-13T10:18:02.967Z
2013-03-13T11:01:57.373Z
2013-03-13T11:01:57.373Z
o0'.
<p>It depends on your risk model.</p>

<p>If you can trust the person paying you, you can accept payment on 0/unconfirmed if you want.</p>

<p>As a merchant or trader, you want to use the configuration that is more secure (no incoming connections permitted, explicity connect to well-connected nodes).</p>

<p>With zero confirmations you are vulnerable to the race attack and the Finney attack, as well as the 51% attack.</p>

<p>With one confirmation you are vulnerable to the 51% attack.  There could also be a miner with a lot of hashing power who could get a couple blocks in a row, so three confirmations removes most of them.</p>

<p>With six confirmations it is essentially mathematically impossible for an attacker with less than 51% of all mining capacity to get six blocks in a row. and still surpass the longest block chain.  With 51% or a lot more than 51% the attacker can get six confirmations by creating a parallel blockchain in which only transactions approved by the attacker get included in blocks.</p>

<p>A merchant like a retailer can likely accept even on 0/unconfirmed in most instances (e.g., purchases up to $100 worth).     A merchant that ships e-commerce might hold off until the transaction has three confirmations.</p>

<p>A cash, face-to-face cash trade will probably be best if three confirmations for a large amount, maybe one confirmation for small amounts. </p>

<p>Any recommendation would need to come only after determining which risks are deemed acceptible.</p>

CC BY-SA 3.0
2013-03-13T13:30:05.543Z
>50% hashrate isn't required to double-spend 6 confirmations. 20% hashrate is enough to have 1% chance, 40% hashrate is enough to have 50% chance.
Meni Rosenfeld
CC BY-SA 3.0
2013-03-14T08:34:26.283Z
I choose this as correct, but I found that there is already a question discussed this a lot. Please refer [Why is 6 the number of confirms that is considered secure?](http://bitcoin.stackexchange.com/q/1170/3176) .
HackFisher
CC BY-SA 3.0
2018-02-19T04:54:14.503Z
@MeniRosenfeld can you provide some reference for these calculations?
Rupsingh
CC BY-SA 3.0
2013-03-13T12:09:05.537Z
2013-03-13T12:09:05.537Z
Stephen Gornick
<p>As a regular user of Bitcoin, I often send bitcoins to other, or receive BTC from others, sometimes this might be a trade or deposit. Some traders require at least 6 confirmation, some require at least 3.</p>

<p>I want to how many confirmation is enough to ensure the transaction is successful?</p>

CC BY-SA 3.0
2013-03-14T08:35:43.990Z
Please refer [Why is 6 the number of confirms that is considered secure?](http://bitcoin.stackexchange.com/q/1170/3176) .
HackFisher
<transactions><confirmations>
How many confirmations do I need to ensure a transaction is successful?
HackFisher
# How many confirmations do I need to ensure a transaction is successful?

(Asked by: username_0 on 2013-03-13)

As a regular user of Bitcoin, I often send bitcoins to other, or receive BTC from others, sometimes this might be a trade or deposit. Some traders require at least 6 confirmation, some require at least 3.

I want to how many confirmation is enough to ensure the transaction is successful?

**Tags:** transactions, confirmations

> **Comments:**
> **username_0**: Please refer [Why is 6 the number of confirms that is considered secure?](http://bitcoin.stackexchange.com/q/1170/3176) .  

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## 3 answers

### Answer by username_3 on 2013-03-13. Score: 18 | Accepted answer

It depends on your risk model.

If you can trust the person paying you, you can accept payment on 0/unconfirmed if you want.

As a merchant or trader, you want to use the configuration that is more secure (no incoming connections permitted, explicity connect to well-connected nodes).

With zero confirmations you are vulnerable to the race attack and the Finney attack, as well as the 51% attack.

With one confirmation you are vulnerable to the 51% attack. There could also be a miner with a lot of hashing power who could get a couple blocks in a row, so three confirmations removes most of them.

With six confirmations it is essentially mathematically impossible for an attacker with less than 51% of all mining capacity to get six blocks in a row. and still surpass the longest block chain. With 51% or a lot more than 51% the attacker can get six confirmations by creating a parallel blockchain in which only transactions approved by the attacker get included in blocks.

A merchant like a retailer can likely accept even on 0/unconfirmed in most instances (e.g., purchases up to $100 worth). A merchant that ships e-commerce might hold off until the transaction has three confirmations.

A cash, face-to-face cash trade will probably be best if three confirmations for a large amount, maybe one confirmation for small amounts.

Any recommendation would need to come only after determining which risks are deemed acceptible.

> **Comments:**
> **username_4**: >50% hashrate isn't required to double-spend 6 confirmations. 20% hashrate is enough to have 1% chance, 40% hashrate is enough to have 50% chance.  
> **username_0**: I choose this as correct, but I found that there is already a question discussed this a lot. Please refer [Why is 6 the number of confirms that is considered secure?](http://bitcoin.stackexchange.com/q/1170/3176) .  
> **username_5**: @MeniRosenfeld can you provide some reference for these calculations?  

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### Answer by username_1 on 2013-03-13. Score: 4

The lesson of the recent v0.7/v0.8 fork issue is that simply waiting for 6 or more confirmations is insufficient. You also need to satisfy yourself that there are no competing blockchain forks

e.g. this thread discusses a double spend proof-of-concept that was executed during the chain fork: <https://bitcointalk.org/index.php?topic=152348.0>


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### Answer by username_2 on 2013-03-13. Score: 0

With 6 confirmations you are safe from double spends and the like.

For low-value transactions, 1 or 2 are likely enough.


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