Proof of work

Proof of work is a method of demonstrating computational effort through a result that is comparatively cheap to verify. In Bitcoin, miners search for acceptable block hashes, and the accumulated work helps nodes choose a transaction history. Satoshi Nakamoto's white paper connects this design to Adam Back's earlier Hashcash system.¹

Hashes and difficultyReport an errorSuggest an improvement

A Bitcoin miner repeatedly changes a candidate block header until its hash meets the network's target. A smaller target makes success less likely per attempt. Finding a solution is probabilistic: a miner can succeed early or search for much longer than average. Other nodes can check the result without repeating the search.²¹

Bitcoin adjusts difficulty every 2,016 blocks, aiming at roughly two weeks for that many blocks. This controls the expected pace as total computing power changes; it does not schedule each block at an exact time.²

Mining and validationReport an errorSuggest an improvement

Full nodes independently enforce Bitcoin's transaction and block rules. They select the valid chain with the greatest cumulative work, rather than simply counting blocks. Mining power therefore does not make an otherwise invalid transaction acceptable to a validating node.²

Miners receive the block subsidy and eligible transaction fees. A mining pool combines participants' work and distributes proceeds under its payout rules. Participants submit easier proofs called shares, allowing the pool to estimate their contributions even when they have not found a network block. Most shares do not meet Bitcoin's network target; those that do can provide a valid block if the other block rules are satisfied.¹³

Security and limitsReport an errorSuggest an improvement

Changing an old block changes its hash and breaks the links to later blocks. An attacker must rebuild that work and compete with the continuing honest chain. In Nakamoto's model, an attacker with less computing power becomes less likely to catch up as confirmations accumulate. This is probabilistic settlement, rather than an absolute guarantee after a fixed number of confirmations.¹

A majority of hash power can reliably compete to replace transaction history, enabling double-spending attacks. Smaller attackers can sometimes succeed too; “51%” is not a threshold below which every attack becomes impossible.²

Mining consumes computing resources and electricity as part of this security mechanism. Ethereum formerly used proof of work, but replaced mining with proof of stake in the Merge on 15 September 2022. Ethereum's documentation estimates that the transition reduced its energy use by about 99.95%; that estimate concerns Ethereum's transition, not all blockchains.¹⁴

See alsoReport an errorSuggest an improvement

• Bitcoin
• Proof of stake
• Ethereum
• Satoshi Nakamoto

ReferencesReport an errorSuggest an improvement