Glossary · Crypto
Proof of work
Proof of work is how some blockchains agree on new blocks: computers must spend real computing effort to add one, and anyone can check that effort instantly.
Also called: PoW, proof-of-work consensus, Nakamoto consensus
Hash rate, 30-day average to November 11, 2026: 1.0 sextillion guesses a second.
In plain words
Try this puzzle: find a number that, written after the word “hello” and run through the SHA-256 hash function, gives a code starting with four zeros. There is no trick. You try 0, 1, 2… and the first that works is 60,067. Checking the answer, though, takes a single try. Hard to find, easy to check: that is proof of work.
Bitcoin uses it to decide who adds the next block. Miners race to find such a number for their block; the winner’s block is accepted because the answer proves the work was done. Every two weeks the puzzle gets harder or easier, so that a winner appears about every 10 minutes.
The work is what protects the record. To change an old payment you would have to redo the work for that block and every block after it, faster than the rest of the network, which makes about 1.0 sextillion guesses a second.
Example
The attacker in Satoshi Nakamoto’s white paper
- An attacker controls 10% of all mining power and wants to undo a payment that already has some blocks on top of it.
- With 1 block on top, the chance he ever catches up with the honest chain is about 20%.
- With 6 blocks on top, about an hour, it is about 0.02%.
- With 30% of the power, 6 blocks still leave him a 13% chance, which is why large payments wait longer.
With more than half of all mining power, the attacker wins sooner or later every time: the “51% attack”.
One level deeper
The idea is older than bitcoin. In 1992 the computer scientists Cynthia Dwork and Moni Naor suggested making email senders do a small calculation to fight spam, and Adam Back’s Hashcash (1997) did it with hashes. Satoshi Nakamoto’s step was to use the work to put transactions in order: the chain with the most work behind it is the true history.
Its strength is also its cost: the security is the electricity and hardware spent. To supporters that is the price of a ledger nobody can cheaply rewrite; to critics it is waste. Ethereum left proof of work for proof of stake in 2022; bitcoin, Litecoin, Dogecoin and Monero still use it.
A small proof-of-work coin is easier to attack, because renting enough machines to out-mine it can be cheap: Ethereum Classic and Bitcoin Gold both suffered 51% attacks in which payments were reversed.
How fast an attacker’s odds shrink
Satoshi Nakamoto’s white paper worked out the chance that an attacker with 10% of the mining power ever catches up with the honest chain, by how many blocks he starts behind. Each extra block cuts the odds about fourfold.
The numbers behind the chart
| Value | |
|---|---|
| 1 block | 20.46% |
| 2 blocks | 5.10% |
| 3 blocks | 1.32% |
| 4 blocks | 0.35% |
| 5 blocks | 0.09% |
| 6 blocks | 0.02% |
Why it matters
It solved the double-spending problem: how to stop digital money from being copied and spent twice without a central clearing house.
It ties security to real-world cost, which makes attacks on a large network extremely expensive.
It is the reason bitcoin uses as much electricity as it does, a cost at the heart of the arguments for and against it.
A common mistake
Myth: Proof of work is wasted effort on useless math.
In fact: The math has no use outside the network, but the spending is the point: it is what makes rewriting history costly. Whether that security is worth the electricity is debated; the work itself is not an accident.
Questions people ask
What is the difference between proof of work and proof of stake?
In proof of work, the right to add a block is won by spending computing power; in proof of stake, by putting coins up as a deposit that can be taken away for cheating. Bitcoin uses work; Ethereum has used stake since 2022.
What is a 51% attack?
When one group controls more than half of a proof-of-work network’s mining power, it can outpace everyone else, rewrite recent blocks and spend the same coins twice. It still cannot take coins from other people’s addresses or change the rules.
Why is it called proof of work?
Because the winning answer is itself the proof: finding it takes, on average, a known amount of computing, while checking it takes a moment.
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About the numbers
- The numbers on this page are recalculated from the data every night; the sources are listed at the end of the page.
- Bitcoin hash rate: Coin Metrics Community Network Data, metric HashRate. License: CC BY-NC 4.0.
- Crypto data: Coin Metrics Community Network Data, used under the CC BY-NC 4.0 license.
- This page explains a term. It is not investment advice.