The difficulty

A dial, not chance

If more machines join, blocks would be found faster. To keep the ten-minute rhythm, the network makes the guessing harder: the threshold the fingerprint must fall below gets smaller.

Every 2016 blocks

The network checks how long the last 2016 blocks took. Too fast and the difficulty rises; too slow and it falls. At ten minutes per block, 2016 blocks are exactly two weeks.

2016 x 10 minutes = 20,160 minutes = 14 days.

Nobody decides this

The adjustment is an arithmetic rule, not a vote. Every computer in the network arrives at the same result independently, because it has the same data and the same formula.

Capped upward and downward

Per adjustment the difficulty can at most quadruple or fall to a quarter. That cap prevents one short-lived outlier from crippling the network for months.

What this means for operators

Rising difficulty lowers the yield per machine without anything changing about the machine. A calculation that only works with today's numbers will not work in half a year.

Historically the difficulty rises far more often than it falls.

The link to hashrate

Difficulty and hashrate move together, but not at the same time: the hashrate changes continuously, the difficulty only every two weeks. In between, blocks run faster or slower than the target.