Bitcoin has a target block time of 10 minutes. Due to the hashing mechanism, it is unknown when a target hash will be achieved; it may be a second or it may be several hours but on average it is aimed to be 10 minutes. The greater the hashing power of the network, assuming a static target difficulty, the quicker a block would be expected to be mined, but due to the randomness of finding a solution the network hashing power is not directly correlated to the block time.
To achieve the 10-minute target block time, the difficulty of the target hash is altered every 2016 blocks, approximately 2 weeks. This re-targeting considers the time taken to mine those 2016 blocks and compares that to the expected time of 20160 minutes (2 weeks). The Bitcoin network then changes the difficulty appropriately; if the 2016 blocks were mined in under 20160 minutes the difficulty target will increase. If the 2016 blocks were mined in over 20160 minutes, the difficulty target will decrease.
The block header contains both the target difficulty and the time the miner reports to have mined the block. The target difficulty is calculated and checked by all verifying nodes on the network. It is therefore considered accurate.
The time, however, is not always accurate, but it is checked that it is greater than the median time of the previous 11 blocks, and not over two hours ahead of the current time as agreed by the connected nodes. In ideal circumstances (11 blocks of exactly 10 minutes each), the median would be 3600 seconds (60 minutes/1 hour). A valid timestamp would therefore be accepted if it were T-60/+120 – a three-hour window, under ideal conditions.
A snapshot of the Bitcoin network’s block headers was taken on 7th November 2022 at block height 762150. The average (mean) block time from the genesis block (3rd January 2009) to block 762150 was 587 seconds; 9 minutes and 47 seconds.
The data was then aggregated by year to give a summary of the Bitcoin’s network performance over that year.
Since 2015, the performance has been impressively consistent; the yearly average (mean) ranged from 563 to 598 seconds; 09:23 to 09:58. In 2010, 2011, 2013, and 2014, the average (mean) time was all under 9 minutes; 07:44, 08:48, 08:17 and 08:55 (respectively). 2012 was an exception during this period with an average block time of 09:39, more aligned with 2015 to 2022 values. Not surprisingly, given the introduction of the Bitcoin network that year and limited adoption, in 2009 the average block time exceeded the target considerably, achieving an average block time of 16:03.
The table below summarises the average (mean) block time per year since Bitcoin’s inception.
| Year | 2009 | 2010 | 2011 | 2012 | 2013 | 2014 | 2015 |
| Mean (Seconds) | 963 | 464 | 528 | 579 | 497 | 535 | 580 |
| Mean (HH:MM:SS) | 00:16:03 | 00:07:44 | 00:08:48 | 00:09:39 | 00:08:17 | 00:08:55 | 00:09:40 |
| Year | 2016 | 2017 | 2018 | 2019 | 2020 | 2021 | 2022 (Partial) |
| Mean (Seconds) | 576 | 563 | 578 | 581 | 594 | 598 | 589 |
| Mean (HH:MM:SS) | 00:09:36 | 00:09:23 | 00:09:38 | 00:09:41 | 00:09:54 | 00:09:58 | 00:09:49 |
Overall, the Bitcoin network has adapted well to the significant increases in hashing power over its lifetime.
The average, however, doesn’t tell the whole picture. Whilst the time required for a 6-block confirmation for transactions may be considered acceptable based on the average block time, on occasions the block time is significantly greater than 10 minutes.
The table below summarises the longest block times in a given year for the past 5 complete years.
| Year | Block | Time | Previous Block Time | Time Difference (HH:MM:SS) |
| 2021 | 689301 | 12:46:53 07/01/2021 | 10:27:39 07/01/2021 | 02:19:14 |
| 2020 | 621701 | 08:39:25 15/03/2020 | 06:46:15 15/03/2020 | 01:53:10 |
| 2019 | 597273 | 17:07:42 30/09/2019 | 15:08:45 30/09/2019 | 01:58:57 |
| 2018 | 554323 | 08:06:20 18/12/2018 | 06:27:32 18/12/2018 | 01:38:48 |
| 2017 | 451858 | 21:10:52 06/02/2017 | 19:31:38 06/02/2017 | 01:39:14 |
These represent the longest blocks, however. Analysis indicates that 95% of blocks will be mined within around 30 minutes of the previous block.
