How many confirmations to be sure about a transaction?

[winguard]

My local exchange coins.ph requires 3 confirmations when cashing out Bitcoin to fiat. So 3 confirmations is sufficient enough.

[ranochigo]

Most transactions don't really need confirmations to be in the <50% risk range. It only needs a few seconds for it to proporgate around the network and ensure that there isn't any competing transactions. One confirmation would be secure enough to accept the transaction unless there is a block reorg. It would be back to unconfirmed. Most exchanges requires over 3-6 confirmations as losses are not a choice for them with huge trade volumes.

[JumperX]

My local exchange coins.ph requires 3 confirmations when cashing out Bitcoin to fiat. So 3 confirmations is sufficient enough.

it really depends on the site if how many confirmation they needed for a transaction to reflect on your balance but atleast 1 confirmation is safe enough and that will not be able to cancel

[pjsonowal]

i think 6 confirmations is enough to get sure about transactions that is 1BTC+.
On other hand 3 confirmations is enough for lower amount of BTCs.

[CasioK]

i think 6 confirmations is enough to get sure about transactions that is 1BTC+.
On other hand 3 confirmations is enough for lower amount of BTCs.

Yes these are the best way of getting your bitcoin transaction most securely. But 6confirmation or 3 confirmations are best suited only with latest bitcoin core client only. If you use old bitcoin-qt client then you must wait for some 30 confirmations.

[ranochigo]

i think 6 confirmations is enough to get sure about transactions that is 1BTC+.
On other hand 3 confirmations is enough for lower amount of BTCs.

Yes these are the best way of getting your bitcoin transaction most securely. But 6confirmation or 3 confirmations are best suited only with latest bitcoin core client only. If you use old bitcoin-qt client then you must wait for some 30 confirmations.

This applies to SPV clients too. They trust the miners without verifying the validity of the blocks.

When pools are SPV mining, they don't check the validity of the block and whether it complies with the network. Those mining pools then generate invalid blocks which get invalidated by Bitcoin core 0.10+ but not anything below or on 0.9.4 or earlier as those Core clients are not aware of the new rules. AFAIK, this isn't a threat right now. Majority of pools are mining on correct rules and creating valid blocks.

[RodeoX]

I think of it like this...

0 confirmations = fast food purchase
3 confirmations = expensive dinner
6 confirmations = cool dirt bike
10 confirmations = 40 acre farm

[VirosaGITS]

i think 6 confirmations is enough to get sure about transactions that is 1BTC+.
On other hand 3 confirmations is enough for lower amount of BTCs.

Yes these are the best way of getting your bitcoin transaction most securely. But 6confirmation or 3 confirmations are best suited only with latest bitcoin core client only. If you use old bitcoin-qt client then you must wait for some 30 confirmations.

This applies to SPV clients too. They trust the miners without verifying the validity of the blocks.

When pools are SPV mining, they don't check the validity of the block and whether it complies with the network. Those mining pools then generate invalid blocks which get invalidated by Bitcoin core 0.10+ but not anything below or on 0.9.4 or earlier as those Core clients are not aware of the new rules. AFAIK, this isn't a threat right now. Majority of pools are mining on correct rules and creating valid blocks.

Which also mean it does not mean if some wallets are outdated and accept some invalid blocks, the network itself will reject it, because most hashrate is pointed at the right wallet version.

Still too bad but not a threat.

[BitcoinBlackjack]

Usually 1 is enough but in the case of confirmations, the more the better!

[ranochigo]

i think 6 confirmations is enough to get sure about transactions that is 1BTC+.
On other hand 3 confirmations is enough for lower amount of BTCs.

Yes these are the best way of getting your bitcoin transaction most securely. But 6confirmation or 3 confirmations are best suited only with latest bitcoin core client only. If you use old bitcoin-qt client then you must wait for some 30 confirmations.

This applies to SPV clients too. They trust the miners without verifying the validity of the blocks.

When pools are SPV mining, they don't check the validity of the block and whether it complies with the network. Those mining pools then generate invalid blocks which get invalidated by Bitcoin core 0.10+ but not anything below or on 0.9.4 or earlier as those Core clients are not aware of the new rules. AFAIK, this isn't a threat right now. Majority of pools are mining on correct rules and creating valid blocks.

Which also mean it does not mean if some wallets are outdated and accept some invalid blocks, the network itself will reject it, because most hashrate is pointed at the right wallet version.

Still too bad but not a threat.

There was a change in block rules after Core version 0.9.5. Clients before that version would accept those blocks blindly so will those using SPV clients. Clients after and on that version will reject the block as invalid as it doesn't follow the rules and refuse to relay it.

A miner generated a block that doesn't follow the rule and miners that verify the blocks do not consider it as valid but those who have SPV mining on will continue mining on that chain and SPV client will follow the longest chain and accept it as valid. This was a threat for merchants accepting 1 confirmation transactions but there wasn't any attack at that time.

[silverleafy]

i think 6 confirmations is enough to get sure about transactions that is 1BTC+.
On other hand 3 confirmations is enough for lower amount of BTCs.

Yes these are the best way of getting your bitcoin transaction most securely. But 6confirmation or 3 confirmations are best suited only with latest bitcoin core client only. If you use old bitcoin-qt client then you must wait for some 30 confirmations.

This applies to SPV clients too. They trust the miners without verifying the validity of the blocks.

When pools are SPV mining, they don't check the validity of the block and whether it complies with the network. Those mining pools then generate invalid blocks which get invalidated by Bitcoin core 0.10+ but not anything below or on 0.9.4 or earlier as those Core clients are not aware of the new rules. AFAIK, this isn't a threat right now. Majority of pools are mining on correct rules and creating valid blocks.

Which also mean it does not mean if some wallets are outdated and accept some invalid blocks, the network itself will reject it, because most hashrate is pointed at the right wallet version.

Still too bad but not a threat.

There was a change in block rules after Core version 0.9.5. Clients before that version would accept those blocks blindly so will those using SPV clients. Clients after and on that version will reject the block as invalid as it doesn't follow the rules and refuse to relay it.

A miner generated a block that doesn't follow the rule and miners that verify the blocks do not consider it as valid but those who have SPV mining on will continue mining on that chain and SPV client will follow the longest chain and accept it as valid. This was a threat for merchants accepting 1 confirmation transactions but there wasn't any attack at that time.

This can happen with any soft-fork, but it only happened once in July when BIP 66 activated. It is possible that that fork issue could happen again if there is another soft-fork where miners don't upgrade and are spv mining, but hopefully they all learned their lesson.

[Corealz]

I have seen alot of people wait between 3-5 transactions so I use that as a rule of thumb for my own transactions.

[favdesu]

something between one and six. If in doubt, wait for three confirmations. that's what I would recommend

[coinyoo]

i would say at least 2 confirmations. But better is to wait until you have 5 confirmations for small amounts. For larger amounts you should wait until at least 20 confirmations.

[bitcoinear]

Thanks for all the answers. Now I have got a good picture of how many confirmations my transactions need in order to be save.

[Miracal]

6 to be absolutely 100% safe.

I agree. for a lesser value transaction a single confirmation has been safe enough. But like its been said for high value transaction, just to be satisfied regarding the risk of reversing, it is better to go for at least five confirmations.

[ranochigo]

6 to be absolutely 100% safe.

I agree. for a lesser value transaction a single confirmation has been safe enough. But like its been said for high value transaction, just to be satisfied regarding the risk of reversing, it is better to go for at least five confirmations.

I wouldn't need a confirmation if it has a small value, convenience over security. Still, it is quite secure already to accept a transaction without confirmations. The risk is pretty much close to 0. Any amount of confirmation wouldn't help if an attacker has over 51% of the total network hashrate for an extended period of time.

[VirosaGITS]

6 to be absolutely 100% safe.

I agree. for a lesser value transaction a single confirmation has been safe enough. But like its been said for high value transaction, just to be satisfied regarding the risk of reversing, it is better to go for at least five confirmations.

I wouldn't need a confirmation if it has a small value, convenience over security. Still, it is quite secure already to accept a transaction without confirmations. The risk is pretty much close to 0. Any amount of confirmation wouldn't help if an attacker has over 51% of the total network hashrate for an extended period of time.

Yes its not really a risk if its like buying a coffee at the starbuck. And one transaction when you are buying something on the internet should not be a big bother if its something like an full priced AAA game.

And the confirmations that "we" recommend to wait before releasing whatever products is not really about the 51% attack. Its more in case we get a rollback because a split or a bug occurred. Some transactions can disappear when that happen, it did in the past, no idea if it will happen again.

[ranochigo]

6 to be absolutely 100% safe.

I agree. for a lesser value transaction a single confirmation has been safe enough. But like its been said for high value transaction, just to be satisfied regarding the risk of reversing, it is better to go for at least five confirmations.

I wouldn't need a confirmation if it has a small value, convenience over security. Still, it is quite secure already to accept a transaction without confirmations. The risk is pretty much close to 0. Any amount of confirmation wouldn't help if an attacker has over 51% of the total network hashrate for an extended period of time.

Yes its not really a risk if its like buying a coffee at the starbuck. And one transaction when you are buying something on the internet should not be a big bother if its something like an full priced AAA game.

And the confirmations that "we" recommend to wait before releasing whatever products is not really about the 51% attack. Its more in case we get a rollback because a split or a bug occurred. Some transactions can disappear when that happen, it did in the past, no idea if it will happen again.

I was referring to the 100% that LFC gave. It kind of sound like it is impossible to double spend after that.

Well, the most serious one was with the fork in 2013. The less serious one was with BIP66 but it wasn't a threat unless you use a SPV client. And yeah, you need 30+ confirmations for it to be safe when using SPV client at that time.

[coinyoo]

This is a great list I bookmarked your post. One question remains. Why 0 confirmations? Wouldn't it be completely risky not waiting until it achieved at least one confirmation?

I think of it like this...

0 confirmations = fast food purchase
3 confirmations = expensive dinner
6 confirmations = cool dirt bike
10 confirmations = 40 acre farm