Definition of confirmation time in blockchain.info, Part 2 In my previous question (Determinant of confirmation time), I asked

"What will be the definition of confirmation time and data used for

https://blockchain.info/charts ?"

Now I read

"The classic bitcoin client will show a transaction as "n/unconfirmed" until the transaction is 6 blocks deep. " (https://en.bitcoin.it/wiki/Confirmation)

Then is it correct to say that, in a simplest approximation it takes 6 times the confirmation time in the graph to get a transaction done?
Then is it correct to say that, in a simplest approximation it takes 6 times the confirmation time in the graph to get a transaction done?


Not quite, the first confirmation is different from all subsequent confirmations, I’ll explain how:

For the first confirmation, there are a couple of factors at play:


how long until the next block is mined
how high the tx fee is, relative to other transactions in the mempool


For all subsequent confirmations, there is just one factor:


how long until the next block is mined. 


The average time between blocks varies, but usually averages just below 10 mins. So a simple approximation of time for 6 confirmations could be:

(Time for first confirmation) + 5*(average block time)

As an example, let’s say we have a tx where the first confirmation takes a while due to a low fee (let’s say, 40 minutes). The next block (next confirmation) will, on average, arrive 10 mins after that, and the next one 10 mins after that. So we might expect the time for 6 confirmations to be (40 + (5*10)) = 90 mins. Whereas under your assumption we would expect it to be (40*6) = 240 minutes. 

It’s worth noting that 6 confirmations is often considered as being ‘fully confirmed’, to help mitigate the risk of a chain reorg happening after one or two confirmations. This number is calculated by considering the cost of an attack to reorg the chain, against the value of the transaction being sent. For high value transactions, you’d want to wait more confirmations, because eventually the cost of an attack to reorg the chain will outweigh the value of the transaction (so a miner would spend more resources attacking the chain than they would gain by reversing the transaction).