Difficulty adjustment

The difficulty adjustment is the automatic mechanism that keeps Bitcoin's blocks arriving about every 10 minutes regardless of how much mining power is on the network. Every 2,016 blocks, roughly two weeks, each node compares how long those blocks actually took with the expected two weeks and raises or lowers the mining difficulty accordingly. A single adjustment is limited to a factor of four in either direction.

If many new miners join and blocks come faster, difficulty rises so each block needs more hashing on average. If miners switch off, as happened when China banned mining in 2021, blocks slow down and the next adjustment makes mining easier.

This self-correcting design is why Bitcoin's issuance schedule stays predictable: the halving every 210,000 blocks lands roughly every four years. Many other proof-of-work coins use similar mechanisms, often adjusting every block. Mining difficulty is also a key input in calculating mining profitability.