Bitcoin: More than an hour to confirm a block

Today it took more than an hour to confirm block 759,054 on Bitcoin’s blockchain. 

As a matter of fact, the previous block was mined at 7:09 UTC, while block 759,054 was mined at 8:34 UTC. 

Thus, for one hour and 25 minutes, Bitcoin’s blockchain stood at block 759,053 waiting for the next block to be mined. 

Mining 759,055, on the other hand, took only 5 minutes. 

Anomaly in the confirmation time of a Bitcoin block

To understand the causes of this apparent problem, it is necessary to know how Bitcoin’s block-time calculation works. 

After all, nowhere does it say in the Bitcoin protocol that about 10 minutes must elapse between each block. The average block time of 10 minutes is just a result of the ratio of difficulty to hashrate. 

Today, for example, the average block time is about 10 and a half minutes, and yesterday it was exactly 10 minutes. So the hour and 25 minutes that it took to mine block 759,054 is not a problem at all, but just a statistical anomaly. 

There is no way to calculate with absolute accuracy how long it takes to mine a block, because in fact the miners proceed at random, and sometimes chance causes it to take much longer to extract the right hash. 

The important thing is not that all blocks are mined 10 minutes after the previous one, but that on average a block is mined every 10 minutes or so. Today’s average is only slightly above this threshold, whereas, for example, Saturday the 15th was 9 minutes and 40 seconds. 

For example, at the end of September several days ended with an average of more than 11 minutes, so everything is absolutely normal. Indeed, such statistical anomalies are actually absolutely normal. 

Hashrate and Bitcoin network difficulty

The really curious thing is that this morning, around 7:00 AM (UTC), there was a small spike in hashrate, which rose to 264.5 Eh/s from 248.8 at 11 PM (UTC) yesterday. So the slowdown in mining of block 759,054 was not due to a drop in hashrate, although this morning at 8:00 AM (UTC) it had dropped to 256.3 Eh/s. Over the rest of the morning, however, it dropped as low as 240 Eh/s, and with no other blocks that slow. 

These are still slightly lower hashrates than in past days, although for example the average daily hashrate on Thursday the 13th was only 234 Eh/s. 

Recently, the difficulty has been growing a lot, with an increase that has been one of the largest ever, bringing it to all-time highs. Then again, the hashrate is also close to all-time highs, recorded last week. 

The increase in difficulty was precisely to lengthen block-time since due to the very high hashrate the average daily block time in early October had dropped below 8 minutes. It remained well under 10 minutes for a good 12 days, due to the very high hashrate, before the sharp increase in difficulty brought it back to around the desired 10 minutes again. 

Therefore, it is more than obvious that as of 10 October, the times to mine Bitcoin blocks have increased a bit, and with a hashrate that is no longer close to 300 Eh/s, the occasional slowdown is to be expected. 

This is well within the norm to such an extent that it doesn’t allow for any really interesting information to be drawn except about the technical workings of Bitcoin. The fact that in the face of all this, the profitability of mining is still dropping, suggests that many miners are mining by accepting little or very little profit in order to collect BTC that they hope will be worth more in the future.