Nonce

The nonce is a 4-byte field in a Bitcoin block header that miners change to produce different hashes while searching for one below the difficulty target. The name is short for number used once. With 32 bits, the nonce offers about 4.3 billion values, which modern hardware exhausts in a fraction of a second.

Why it matters

The nonce embodies proof of work's asymmetry: finding a valid one requires quintillions of guesses network-wide, while verifying it takes a single hash by anyone. That asymmetry lets a block's validity be checked by a phone even though producing it consumed industrial energy. The costly search anchored by the nonce is bitcoin's version of gold's mining cost, the unforgeable expense that keeps supply honest. Skeptics call the expenditure waste; proponents reply that gold's monetary credibility rested on the same principle, that money no one can cheaply produce is money worth holding.

How it works

Mining is repeated hashing of the 80-byte block header. Each nonce value yields an unpredictable hash, and almost all fall above the target and are discarded. Because ASICs now compute hundreds of trillions of hashes per second, the nonce space alone is far too small, so miners also vary an extra nonce in the coinbase transaction and roll the timestamp, changing the Merkle root and opening fresh search space.