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What is proof of work?
Proof of work is a consensus mechanism in which miners compete to solve a mathematical puzzle by trial and error, spending computing power and electricity. The winner adds the next block. Bitcoin uses it; Ethereum used it until 2022.
What it means for you
Rewriting a proof-of-work chain takes a majority of the network's computing power, so each block added after your payment makes it more costly to reverse. On a smaller chain with less computing power behind it, that majority costs less to assemble.
A common mistake: “Once my transaction has one confirmation, it can't be reversed.”
In fact: Under proof of work a recent block can be replaced by a competing chain with more work. Each block added on top makes that less likely, which is why the Bitcoin FAQ cites six confirmations as a common threshold.
How it works
Miners hash a block header with a changing nonce until the result is below a target; the expected work grows exponentially as the target tightens, yet anyone can check a solution with one hash. Nodes follow the chain with the most accumulated work, so rewriting a block means redoing its work and that of every later block, faster than the honest majority. Finality is therefore probabilistic: each block on top makes reversal less likely, and the Bitcoin whitepaper shows an attacker's odds of catching up fall exponentially with each block behind. Proof of stake instead puts staked coins at risk and has explicit finality.
Sources: ethereum.org: Proof-of-work, ethereum.org: Consensus mechanisms, Bitcoin whitepaper (PDF), Satoshi Nakamoto, Bitcoin developer guide: Block chain, bitcoin.org: Frequently Asked Questions · checked 4 October 2026
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