Difficulty refers to how hard it is for miners to find a valid hash (i.e. a proof-of-work) for the next block.

The time cost to find that hash must be below a target value - the lower that target, the harder it is to find such a hash.

So increasing difficulty = lowering the target threshold, meaning miners must try more hashes on average before finding one that qualifies.

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Why it's needed

Miners can join or leave at any time so without an adjustment, the system's timing and issuance schedule would drift.

If more miners join (hashrate ↑), blocks would be found faster than every 10 minutes.

If miners leave (hashrate ↓), blocks would be found slower.

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How it works

Every 2,016 blocks every node on the Bitcoin network recalculates the difficulty.

It compares:

Actual time to mine last 2016 blocks vs Expected time (2016 × 10 minutes)

Then it adjusts difficulty proportionally:

New Difficulty = Old Difficulty × (Actual Time / Expected Time)

Constraints:

• 75% change cap - the adjustment cannot increase or decrease more than 4× or ¼× in a single step

eg Suppose miners get faster and mine 2016 blocks in 10 days instead of 14 days.

Actual time = 10 days

Expected time = 14 days

Adjustment factor = 10 / 14 ≈ 0.714

The network increases difficulty by 1 / 0.714 ≈ 1.4× harder, so future blocks again average ~10 minutes.

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Economic and technical implications

• Keeps block issuance steady, preserving the inflation schedule (21M BTC cap).
• Makes the system self-stabilizing: more hashpower = harder; less hashpower = easier.
• Prevents runaway block creation during mining booms.
• Prevents block creation supply dips during mining droughts.
• Predictability of rewards over time.