Target

The target is used in mining. It is a number that a block hash must be below for the block to be added on to the blockchain.

The target adjusts every 2016 blocks (roughly two weeks) to try and ensure that blocks are mined once every 10 minutes on average. It therefore creates a consistent time between blocks and a consistent issuance of new bitcoins in to the network (via the block reward).

To compensate for increasing hardware speed and varying interest in running nodes over time, the proof-of-work difficulty is determined by a moving average targeting an average number of blocks per hour. If they’re generated too fast, the difficulty increases. – Satoshi Nakamoto

Current target:

The target is in hexadecimal, but it’s still a number.

1. When does the target adjust?

The target adjusts on every 2016th block. This is roughly every two weeks (because there are 20,160 minutes in a two-week period).

Here are the dates and target values for every target adjustment in Bitcoin’s history:

Bitcoin Target History

2. How is the target calculated?

The target for the first block was set at:

This initial (and maximum) target value is hard-coded in to the source code of every bitcoin node.¹ It was probably best-guess by Satoshi at what be a good starting point for a difficult-enough target that would result in a 10-minute interval between new blocks.

After every 2016 blocks, each node will look at the time between the last 2016 blocks and work out if they were being mined faster or slower than 10 minutes on average.

Each block contains a timestamp in its block header. This timestamp is in Unix time, which is the number of seconds since 01 January 1970.

If blocks during this period were being mined faster than every 10 minutes, the target will adjust downwards to make it more difficult to get below the target for the next period of blocks.

Conversely, if blocks were being mined slower than every 10 minutes, the target will adjust upwards to make is easier to get below the target for the next period of blocks.

As a result, each node regularly recalculates the target to enforce a 10-minute interval between new blocks, even as miners join and leave the network over time.

See the code at the bottom of the page for a real example.

What causes blocks to be mined faster or slower than every 10 minutes?

Well, first of all, mining is unpredictable, so you never know when a miner is going to find the next block with a block hash below the current target.

Secondly (and most importantly), miners can join and leave the network at any time, which affects the likelihood of new blocks being mined within 10-minute periods. The more miners there are on the network, the more hashing there is taking place, and the more likely it is a new block will be mined in under 10 minutes.

3. Do all nodes share the same target?

Each node on the network operates independently, so there is no “single target value” being sent around the network.

However, because all nodes adopt the longest chain of blocks as their blockchain, they will each end up calculating the same target as everyone else, so effectively all nodes end up sharing the same current target.

For example, when you run bitcoin for the first time, your node will perform the initial block download and calculate the targets as it goes. And because you’re receiving the same blocks as everyone else, you will end up calculating the same current target too.

Furthermore, all nodes continually share the same view of the blockchain (as they will always adopt the longest available chain of blocks). Therefore, nodes will also continually calculate the same target value as each new block is mined.

As a result, despite the fact nodes calculate the target independently, they each share the same blockchain and calculate the same target value as each other.

Everyone does the same calculation with the same chain data, so they all get the same result at the same link in the chain. – Satoshi Nakamoto

4. Why does bitcoin use a target?

The target regulates the speed at which new blocks are added on to the blockchain.

This has two benefits:

i. It gives blocks time to propagate across the network.

It’s ideal to have miners working on extending the same chain of blocks as much as possible. For this to work, we need to allow time for new blocks to propagate across the network before the next block gets mined.

If blocks are being mined faster than they can be broadcast across the network, it will result in miners regularly building competing chains. Only one of these will become the longest, so some miners will end up wasting energy working to build on top of a competing chain only for it to be left behind due to a chain reorganisation.

Therefore, this time delay between blocks allows them to propagate the network so that more miners can adopt the longest available chain, which concentrates the network’s mining power on extending the same chain of blocks.

If broadcasts turn out to be slower in practice than expected, the target time between blocks may have to be increased to avoid wasting resources. We want blocks to usually propagate in much less time than it takes to generate them, otherwise nodes would spend too much time working on obsolete blocks. – Satoshi Nakamoto

ii. A consistent issuance of new bitcoins.

Bitcoin is a currency, so having a fixed rate of new bitcoins introduced in to the system provides stability.

Coins have to get initially distributed somehow, and a constant rate seems like the best formula. – Satoshi Nakamoto

5. Why 10 minutes between blocks?

I don’t think anyone knows why 10 minutes exactly was chosen, except for Satoshi.

My guess is that it seems like a long-enough time to allow blocks to propagate around the network (to minimize chain reorganisations), and short-enough so that you do not have to wait too long for a new transaction to get mined on to the blockchain. And 10 is a nice round number.

6. Where can you find the target?

The target gets stored in the bits field in the block header of every block.

Commands

$ bitcoin-cli getblocktemplate '{"rules": ["segwit"]}' | grep target
  "target": "0000000000000000000730390000000000000000000000000000000000000000",

$ bitcoin-cli getdifficulty
43551722213590

$ bitcoin-cli getblockheader 000000000000000002e9533a4fe03bb251b3fdb30ffaa384aad133b7fae594cf
{
  "hash": "000000000000000002e9533a4fe03bb251b3fdb30ffaa384aad133b7fae594cf",
  "confirmations": 207763,
  "height": 401184,
  "version": 4,
  "versionHex": "00000004",
  "merkleroot": "b4afa0502a55fdfa4a5cda6ea2bc546ba527d276ea9c7e848b8cd478cd9b6607",
  "time": 1457133956,
  "mediantime": 1457132303,
  "nonce": 3778923481,
  "bits": "1806f0a8",
  "difficulty": 158427203767.3917,
  "chainwork": "00000000000000000000000000000000000000000012da32364c8f47d519604d",
  "nTx": 845,
  "previousblockhash": "000000000000000003c4a4d9c62b3a7f4893afe14eef8a6a377229d23ad4b1ea",
  "nextblockhash": "00000000000000000037ab74c8db9b7a9d7de039210d8eafaeb44ba35adfb624"
}

Code

Here’s some example Ruby code for calculating new target values. This code works out the new target for block 403,200, using the timestamps from blocks 401,184 and 403,199.

# 403,200 - NEW TARGET
# 403,199              | last block
#                      |
#                      |
#                      |
# 401,184 - NEW TARGET | first block (target = 0x000000000000000006f0a8000000000000000000000000000000000000000000)

# 1. Get the timestamps for the first and last block in the target adjustment period
first = 1457133956 # block 401,184
last  = 1458291885 # block 403,199

# 2. Work out the ratio of the actual time against the expected time
actual = last - first     # 1157929 (number of seconds between first and last block)
expected = 2016 * 10 * 60 # 1209600 (number of seconds expected between 2016 blocks)
ratio = actual.to_f / expected.to_f

# 3. Limit the adjustment by a factor of 4 (to prevent massive changes from one target to the next)
ratio = 0.25 if ratio < 0.25
ratio = 4 if ratio > 4

# 4. Multiply the current target by this ratio to get the new target
current_target = 0x000000000000000006f0a8000000000000000000000000000000000000000000
new_target = (current_target * ratio)

# 5. Don't let the target go above the maximum target
max_target = 0x00000000ffff0000000000000000000000000000000000000000000000000000
new_target = max_target if new_target > max_target

# 5. Truncate the target, because the official target is the truncated "bits" format stored in the block header
# This code is a bit rough, because it's working with strings when I should really be working with actual bytes.
new_target = new_target.to_i.to_s(16) # convert from decimal to hexadecimal
new_target = new_target.size % 2 != 0 ? '0' + new_target : new_target # make sure it's an even number of characters (i.e. bytes)
truncated = new_target.scan(/../).each_with_index.map { |byte, i| byte = i >= 3 ? "00" : byte }.join # set all bytes apart from first 3 to zeros
# e.g. 6a4c316c01f354000000000000000000000000000000000 <- full precision
# e.g. 6a4c3000000000000000000000000000000000000000000 <- official target

# 6. Display the full target (with leading zeros)
target = truncated.rjust(64, '0')
puts target
# 000000000000000006a4c3000000000000000000000000000000000000000000

Tip: You can get the timestamp for a block with bitcoin-cli getblockheader <hash> (and you can get the block hash for a block at a specific height with bitcoin-cli getblockhash <height>).

Official Target: The official target is the truncated target that gets stored in the block header (in the bits field), so it’s not actually the full-precision target that you get after adjusting the previous one.

Off-by-one: You may notice that new target is calculated using the time across 2015 blocks (and not across 2016 as you would expect). This is an implementation error in the code that still exists today.

Factor of 4: Target adjustments are limited by a factor of 4 to prevent overly-large adjustments from one target to the next.

Links

• pow.cpp - The function CalculateNextWorkRequired() adjusts the target.
• chainparams.cpp - The original target value (and also the highest possible target value) is set by consensus.powLimit
  
  
• https://en.bitcoin.it/wiki/Target
• How is the target in Bitcoin set? Who does this? (bitcoin.stackexchange.com)
• Why not retarget on every block? (bitcoin.stackexchange.com)
• Why was the target block time chosen to be 10 minutes?
• https://bitcoin.stackexchange.com/questions/1511/gaming-the-off-by-one-bug-difficulty-re-target-based-on-2015-instead-of-2016)

Thanks

• Thanks to David Harding for pointing out that you can get the target directly from bitcoind with bitcoin-cli getblocktemplate.