Bitcoin's difficulty

Bitcoin's difficulty is a number that says how hard it is to add the next block to the Bitcoin ledger. It compares the current target with a fixed reference target, and it is recalculated every 2,016 blocks so that a new block is found about every ten minutes.¹

Target, hash and nonce

A block is a batch of transactions with an 80-byte header. The header holds a reference to the previous block, a summary of the transactions, a time, the current target in compressed form, and a 4-byte field called the nonce.² The header is hashed twice with SHA-256 (see the note on SHA-256). Read as a 256-bit number, the result must be less than or equal to the target.²

There is no way to calculate a header that meets the target. The only way is to guess: change the nonce, which is there to be changed, hash again, and repeat until a result happens to be small enough.^3, 4 A small number written out in 64 hexadecimal digits begins with many zeros, which is why the rule can also be put as a hash that "starts with a required run of zeros".⁴ The block at height 972,231, found on 20 October 2026, has a hash that begins with 19 zeros.⁵

What the number means

The Bitcoin Wiki defines the difficulty as difficulty_1_target / current_target: how many times smaller the target is now than at difficulty 1.¹ The same page gives the expected number of hashes needed for one block as D × 2⁴⁸ / 0xffff, where D is the difficulty, which is close to D × 2³².¹ On 16 October 2026, at block 971,712, the difficulty was set to about 140.6 trillion.⁶ By the formula, that is about 6 × 10²³ guesses, on average, for every block.

The adjustment

Every 2,016 blocks, the network uses the timestamps in the block headers to see how long the last period took. The ideal is 1,209,600 seconds, two weeks, which is ten minutes per block.³ If the blocks came faster, the difficulty goes up in proportion, by at most 300 %; if they came slower, it goes down, by at most 75 %.³ The adjustment of 16 October 2026 raised the difficulty by about 5.9 %.⁶

The difficulty therefore follows the computing power spent on the whole network, not anything inside a single block. Every block within the same period of 2,016 blocks has the same difficulty.¹

Where to see it

Olof Broström's Difficulty is drawn from these blocks. Each day at 00:30 UTC, JAEV's server makes one drawing from the day's aurora forecast and the first block after midnight UTC: the block's hash sets the background colours and the angle of the lines, the number of transactions sets the number of lines, from 100 to 260, the nonce sets how high they are lifted, and the block's height becomes the sheet number.⁴ The drawing for 20 October 2026 is Difficulty no. 972231.⁴