Target

The number you need to get below to mine a block

Diagram showing the block hash of a candidate block trying to get below a target value.
Current Target:
Height: 849,259 (the upcoming block)
000000000000000000035d250000000000000000000000000000000000000000

The target is a number that a block hash for a candidate block must get below before the block can be added on to the blockchain.

It is used during the process of mining.

The target adjusts every 2016 blocks (roughly two weeks) to try and ensure that blocks are mined once every 10 minutes on average. This creates a consistent time between blocks, and a consistent issuance of new bitcoins (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, Bitcoin Whitepaper

Period

When does the target adjust?

The target adjusts every 2016 blocks.

This is roughly every two weeks.

Blocks are mined roughly every 10 minutes, and there are 20,160 minutes in two weeks.

History

Here are the block heights and target values for every target adjustment in Bitcoin's history:

Height Target Adjustment* Time
850752 Next adjustment in 1493 blocks ()    
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122976 0000000000006a93b30000000000000000000000000000000000000000000000 x 0.69668809608166
120960 00000000000098fa000000000000000000000000000000000000000000000000 x 0.84204867979702
118944 000000000000b5ac000000000000000000000000000000000000000000000000 x 0.89169239028318
116928 000000000000cbbd000000000000000000000000000000000000000000000000 x 0.83766160764474
114912 000000000000f339000000000000000000000000000000000000000000000000 x 1.1045964980752
112896 000000000000dc31000000000000000000000000000000000000000000000000 x 0.72959190277113
110880 0000000000012dcd000000000000000000000000000000000000000000000000 x 0.65587700979643
108864 000000000001cc26000000000000000000000000000000000000000000000000 x 0.71305432137625
106848 0000000000028552000000000000000000000000000000000000000000000000 x 0.84669909232086
104832 000000000002fa29000000000000000000000000000000000000000000000000 x 0.83760335190734
102816 0000000000038dee000000000000000000000000000000000000000000000000 x 0.8844633615698
100800 00000000000404cb000000000000000000000000000000000000000000000000 x 0.88819454749025
98784 000000000004864c000000000000000000000000000000000000000000000000 x 0.84588942424268
96768 0000000000055953000000000000000000000000000000000000000000000000 x 0.65933687691144
94752 0000000000081cd2000000000000000000000000000000000000000000000000 x 0.85005356143577
92736 0000000000098b2a000000000000000000000000000000000000000000000000 x 0.66061171937548
90720 00000000000e7256000000000000000000000000000000000000000000000000 x 0.68154669565311
88704 0000000000153263000000000000000000000000000000000000000000000000 x 0.69508560757914
86688 00000000001e7eca000000000000000000000000000000000000000000000000 x 0.64123507516441
84672 00000000002f8e9d000000000000000000000000000000000000000000000000 x 0.95692532533823
82656 000000000031b2a3000000000000000000000000000000000000000000000000 x 0.69602759325171
80640 00000000004766ed000000000000000000000000000000000000000000000000 x 0.77670624056589
78624 00000000005bede6000000000000000000000000000000000000000000000000 x 0.87445671926363
76608 0000000000692098000000000000000000000000000000000000000000000000 x 0.82095643188335
74592 0000000000800e00000000000000000000000000000000000000000000000000 x 0.68811922753988
72576 0000000000ba1800000000000000000000000000000000000000000000000000 x 0.69346997001368
70560 00000000010c5a00000000000000000000000000000000000000000000000000 x 0.74338026035298
68544 000000000168fd00000000000000000000000000000000000000000000000000 x 0.25
66528 0000000005a3f400000000000000000000000000000000000000000000000000 x 0.51781054106111
64512 000000000ae49300000000000000000000000000000000000000000000000000 x 0.82569184041232
62496 000000000d314200000000000000000000000000000000000000000000000000 x 0.8957315430679
60480 000000000eba6400000000000000000000000000000000000000000000000000 x 0.9561367013373
58464 000000000f675c00000000000000000000000000000000000000000000000000 x 0.71280895449038
56448 00000000159c2400000000000000000000000000000000000000000000000000 x 1.084664501261
54432 0000000013ec5300000000000000000000000000000000000000000000000000 x 0.89222137108815
52416 0000000016546f00000000000000000000000000000000000000000000000000 x 0.68209888195793
50400 0000000020bca700000000000000000000000000000000000000000000000000 x 0.77821418013446
48384 000000002a111500000000000000000000000000000000000000000000000000 x 0.7501734015021
46368 0000000038137500000000000000000000000000000000000000000000000000 x 0.99253451389671
44352 00000000387f6f00000000000000000000000000000000000000000000000000 x 0.83449813104228
42336 0000000043b3e500000000000000000000000000000000000000000000000000 x 0.66850525896459
40320 0000000065465700000000000000000000000000000000000000000000000000 x 0.71947661561676
38304 000000008cc30000000000000000000000000000000000000000000000000000 x 0.73913400201013
36288 00000000be710000000000000000000000000000000000000000000000000000 x 0.97086585948702
34272 00000000c4280000000000000000000000000000000000000000000000000000 x 0.90639327100105
32256 00000000d86a0000000000000000000000000000000000000000000000000000 x 0.84538033112078
30240 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
28224 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
26208 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
24192 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
22176 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
20160 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
18144 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
16128 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
14112 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
12096 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
10080 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
8064 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
6048 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
4032 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
2016 00000000ffff0000000000000000000000000000000000000000000000000000 x 1
0 00000000ffff0000000000000000000000000000000000000000000000000000  

* The adjustment ratios in this table are calculated based on the change from one target to the next. The actual adjustment ratios are calculated based on the time between blocks. See each block for details.

Adjustment

How is the target calculated?

tool-66791044e973e
Tool Icon

Target Adjustment

Calculate the next target value based on the current target and the time between blocks.

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Time (seconds)
0d
0d

The target adjustment period is 2016 blocks. A block is mined on average every 600 seconds (10 minutes), so the expected time is 2016 * 600 = 1209600 seconds.

Ratio

The actual time divided by the expected time. We multiply the current target by this ratio to get the new target.

0x
0x
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Note: This target value has been truncated slightly for storage in the bits field of the block header, and that's the target value that's actually used when mining.

0 secs

The target for the first block was set at:

Height: 0 (Genesis Block)
00000000ffff0000000000000000000000000000000000000000000000000000

Diagram showing the target for the genesis block.

This initial target value is hard-coded in to the source code of every Bitcoin node (see chainparams.cpp). It's also the maximum possible target value. It was most likely a 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.

On every 2016th block, each node will look at the time between the previous 2015 blocks and work out if they were mined faster or slower than 10 minutes on average.

Diagram showing the target period between the genesis block and the 2016th block along with the timestamp of each block.

Each block contains a timestamp in its block header. This timestamp is in Unix Time, which is the number of seconds since .

If blocks during this period were 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.

Diagram showing the target value adjusting downwards due to blocks being mined more quickly than expected.

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

Diagram showing the target value adjusting upwards due to blocks being mined more slowly than expected.

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

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

Synchronisation

How do nodes calculate the same target?

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, Bitcoin P2P e-cash paper

Each node on the network operates independently, so there is no central authority to determine the current target value.

However, because nodes always adopt the longest chain of blocks as their blockchain, they will each calculate the same 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 when you reach the tip of the blockchain.

Diagram showing the initial block download and target calculations.

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

Diagram showing nodes on the network recalculating the target when new blocks are 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.

Purpose

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:

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

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, Bitcoin P2P e-cash paper

It's preferable 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.

Diagram showing a newly-mined block propagating across nodes on the network.

If blocks are being mined faster than they can be broadcast across the network, it will result in miners regularly working on top of "old" blocks in the blockchain (because they haven't had chance to receive the latest blocks yet).

This will result in miners building multiple competing blockchains; 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.

Diagram showing miners working on building two different blockchains due to not receiving the latest blocks quickly enough.

Therefore, this time delay (block interval) allows blocks to propagate the network so that more miners can adopt the longest available chain, which helps to concentrate the network's mining power on extending the same chain of blocks.

Diagram showing miners focused on extending the same blockchain due to the latest block having time to propagate the network.

2. A consistent issuance of new bitcoins.

Coins have to get initially distributed somehow, and a constant rate seems like the best formula.
Satoshi Nakamoto, Bitcoin P2P e-cash paper

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

Diagram showing block rewards being issued at a constant rate.

So thanks to the target, you can be confident that new bitcoins will be minted at a predictable rate.

Location

Where can you find the target?

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

Diagram showing the target being stored in the bits field of a block header.

The bits field is a compact representation of the target (to save space in the block header). But you can easily convert between the bits representation and the full target.

Target Bits

Commands

bitcoin-cli getblocktemplate

This is the simplest way to get the current target. When you request a block template (for mining purposes) it will also return the current target.

This command returns a lot of data, so it's best to grep for the target.

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

"target": "000000000000000000035d250000000000000000000000000000000000000000",

bitcoin-cli getdifficulty

Alternatively, to get the current target you can ask for the current difficulty, which you can then convert to the target:

$ bitcoin-cli getdifficulty
83675262295059.91
Difficulty

Converting from difficulty to target is not always accurate. The difficulty is a floating point number calculated from the change between the first and current target value, so it can lose precision and give you an inaccurate result.

bitcoin-cli getblockheader [block hash]

This command allows you to find a previous target for specific block. The target at the time of mining is stored in every block header in bits format, which you can convert to a full target value.

$ bitcoin-cli getblockheader 000000000000000002e9533a4fe03bb251b3fdb30ffaa384aad133b7fae594cf
{
  "hash": "000000000000000002e9533a4fe03bb251b3fdb30ffaa384aad133b7fae594cf",
  "confirmations": 425677,
  "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"
}
Target Bits

You can get the target for the most recent block in the chain using this one-liner:
$ bitcoin-cli getblockheader $(bitcoin-cli getblockhash $(bitcoin-cli getblockcount))

Code

Here's some example Ruby code for calculating target adjustments.

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

Off-by-one error. Target adjustments are actually calculated using the time across 2015 blocks (e.g. 403199 - 401184 = 2015), and not across 2016 blocks as you would expect. This is an implementation error in the code, and it still exists today.

Official Target. The actual target that miners need to get below when mining is the truncated target that gets stored in the bits field. So it's not actually the full-precision target that you get after a target adjustment calculation.

FAQs

Why 10 minutes between blocks?

I don't think anyone except Satoshi knows why 10 minutes was chosen exactly.

My guess is that it seemed long enough to allow blocks to propagate around the network (to minimize chain reorganisations), and short enough to not have to wait too long new transactions to get mined on to the blockchain.

And 10 is a nice round number.

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

Firstly, 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 speed at which blocks can be mined. For example, the more miners that join the network, the more hashing there will be taking place, and the more likely it is a new block will be mined in under 10 minutes.

Resources

Thanks