Why is the block time on bitcoin 10 mins? An introduction to PoW Mining(Part 1)

The block time on the bitcoin blockchain is 10 minutes.

This means that every 10 minutes a new block of transactions is added to the blockchain and transactions within the block are considered to be ‘processed’. However this block time isn’t set in stone and can oscillate between a few seconds and a few days!

Block Time

The longest ever wait between bitcoin blocks was between the genesis block (the first block on the bitcoin blockchain) and block 1; a huge 5 days, 8 hours and 32 minutes. This extended wait was due to limited computational power on the network since Satoshi was the only miner, and the challenge of long delays between blocks continued throughout bitcoin’s first year as is visible on the chart below.

However since this initial teething period, the daily average wait time has stabilised with the most recent record being just 122 minutes for block 679,786 to be mined.

As well as the gap between blocks being over the 10 minute ideal, it’s also possible for two blocks to be found at exactly the same time or within a very short timeframe of each other. In the case that two blocks are found at the same time there is a temporary fork in the network with two blocks sharing the same block height.

https://www.linkedin.com/pulse/whats-difference-between-blockchain-network-split-fork-tara-annison/

Miners must then choose which side to build their candidate block on top of and eventually one side will become longer. Then, following what’s known as “Satoshi’s Rule” where the longest chain wins, this becomes the official bitcoin blockchain and the block/s in the losing chain are orphaned; with any transactions not also included in the main chain being returned to node mempools.

https://tara-annison.medium.com/what-is-a-mempool-f6a403d4871e

However it’s also possible for two blocks to be found in quick succession and therefore have sequential block heights. This was the case with blocks 697,095 and 697,096 which were found within a minute of each other and therefore share the timestamp of 2021–08–23 01:48 even though they are consecutive blocks.

How then does the bitcoin blockchain try to keep to a regular 10 minute interval? Unsurprisingly this isn’t magic or someone from the bitcoin core dev team pushing a button, instead this is some clever code within the bitcoin protocol itself…

Difficulty

Imagine three people are given a combination padlock which is locked.

Tony may start with the value 000 and then try 001, 002 ….

Diana may start with the value 999 and then try 998, 997

Scott might try random combinations.

Assuming the correct combination is 123 then it’s likely that Tony will be able to unlock the padlock first — unless Scott has luck on his side. However assuming our group are lightning quick at moving through the possible combinations, you might want to slow them down by adding more than 3 digits on the combination lock. You therefore increase the difficulty based on the computational power of the network.

This is what’s happening on the bitcoin blockchain every 2016 blocks (every approx. 2 weeks) where a calculation is done to assess how long it took to find the last 2015 blocks (this is a little quirk in the system introduced by a bug) and if this is above the 10 minute target then the padlock is made easier, and if it’s below then the padlock is made harder. This is referred to as a difficulty adjustment and helps keep the average time between blocks at 10 minutes.

There are small adjustments being made after each difficulty epoch since the computational power on the network fluctuates. However the most notable change in network difficulty was during the summer of 2021 when China announced that it was banning mining activity in the country. As a result, mining firms switched off their machines and scrambled to move them to other territories. This lead to a temporary drop in hashrate on the network and through the next difficulty adjustments the network self regulated by dropping the difficulty by a whopping 28% — the biggest downward adjustment in the history of the network. Then as mining firms resumed their activity in their new locations, the difficulty steadily adjusted back upwards.

However miners on the bitcoin blockchain aren’t trying to solve padlocks, they’re doing some maths. So in part 2 we look at what’s going on under the hood with difficulty from a mathematical perspective.

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Tara Annison

Tara Annison

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