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Agreement (consensus) · 2 min read

What is proof of work?

By Oliver James Whitmore · Updated 20 October 2026

In one sentence

Proof of work is a consensus method in which the right to add the next block goes to whoever first finds a block hash below a target — something that takes enormous computation to find but a single calculation to check.

Proof of work as a target: each try is a random point; only hashes below the target count.valid0largest possible hashEvery hash attempt lands somewhere at randomDifficulty = how narrow the valid zone is. Checking a result takes one hash.
Proof of work as a target: each try is a random point; only hashes below the target count.

How it works

The idea is older than blockchains. In 1992 Cynthia Dwork and Moni Naor proposed making senders do a small computational task before each e-mail, to make mass spam expensive. Adam Back's Hashcash, in 1997, used hashing for the same purpose. The Bitcoin whitepaper of 2008 applied it to the question of who adds the next block.

Miners repeatedly hash block headers, changing the nonce each time, until one result falls below the current target. The chance of success on any single try is tiny, so the time to find a block depends on the total computing power of the network. To keep blocks at about one every ten minutes, Bitcoin recalculates the target every 2,016 blocks, roughly every two weeks.

Nodes follow the valid chain that represents the most accumulated work. To rewrite a past block, an attacker would have to redo the work for that block and every block after it, and then overtake the rest of the network — which in practice requires controlling a majority of all computing power.

The security has a real cost: specialised hardware and a large amount of electricity. Ethereum used proof of work until 15 September 2022, when it switched to proof of stake.

A simple example

Think of a lottery in which every ticket costs a little electricity, and the winning ticket is printed with a mark that anyone can check at a glance. Buying more tickets raises your chance of winning a round; it never lets you print a winning ticket for a round that has already been decided by others.

Common misconception

Misconception: Proof of work decides which transactions are valid.

In fact: Validity is checked by every node against the rules, independently of the work. Proof of work only decides who gets to propose the next block and which valid history wins. An invalid block is rejected however much work it carries.

Where you meet it

  • Bitcoin and a number of other networks.
  • Statistics on "hash rate", the total guesses per second.
  • Debates about the energy use of blockchains.

Sources

This card explains a technical concept for general education. It is not a recommendation to acquire, sell or hold any crypto-asset, and it is not financial advice.