Nonce
TL;DR
A nonce is a 32-bit number in the Bitcoin block header that miners vary to find the right hash during mining.
The nonce is the variable miners manipulate during SHA-256 brute force. 2^32 = ~4.3 billion values. At modern hashrates that's exhausted in microseconds; miners also vary the 'extranonce' in the coinbase transaction to effectively do 2^64+ attempts per block candidate.
Example
A miner generates a block header with candidate transactions. Tries nonce=0: hash too high. nonce=1: too high. ... nonce=3,271,455,829: hash below difficulty! Block found.
Frequently asked questions
Can I predict the nonce?
No. SHA-256 is pseudo-random; finding a valid nonce is brute force.
Related terms
Further reading
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