Nonce

The nonce (usually explained as ‘number used once’) is a 4-byte field in the header of every Bitcoin block. In proof of workGlossaryProof of workThe method by which miners prove that they have done real computing work for a new block. It lets the network agree on the valid chain without a central authority, and makes rewriting old blocks extremely expensive.On the learning path: Stage 1 · Step 3 – How does Bitcoin work? →In the glossary →×, it is the dial that miners turn.

What the nonce is for

A block is only valid if the hashGlossaryHash (hash value)A fixed-length digital fingerprint that a hash function calculates from any data. Even the tiniest change produces a completely different value, and the original data can’t be worked out from the hash.On the learning path: Stage 1 · Step 3 – How does Bitcoin work? →In the glossary →× of its header is less than or equal to a target. So miners change the nonce and compute the hash again: each value gives a completely different hash, and therefore a new attempt.^[1]

The right nonce can’t be predicted, only found by trial and error. The more a miner computes, the better their chances – but there’s no guarantee. Checking a find, on the other hand, takes any node a single hash calculation.

When the values run out

4 bytes are 32 bits, or around 4.3 billion possible values. A mining device with an assumed 200 TH/s (200 trillion hashes per second) would run through them in about 21 microseconds (own calculation). Miners then change the timestamp or a free field in the block’s first transaction (the ‘extra nonce’), which changes the Merkle root in the block header. After that, the search starts again.^[1]

More under Mining & proof of work.