Proof of work means a block's hash is so rare that finding it took huge effort, yet anyone can check it instantly. See how it secures Bitcoin.
Hard to make, easy to check
A hash function produces unpredictable output. To get a hash below a certain number, there's no shortcut: you must try again and again. But once someone shows you the winning input, you can check it with one hash.
That's proof of work: the block hash itself proves that, on average, a huge number of attempts were made.
Why it secures Bitcoin
- Each block's header includes the previous block's hash, so the work stacks up.
- To change an old block, an attacker must redo its work and every block after it, then overtake the honest network.
- Honest miners keep adding blocks meanwhile, so the attacker falls further behind unless they control most of the hash power. See The 51% attack.
Try it yourself
Bitcoin's real target needs a hash starting with about 19 or more hex zeros. That's why miners use ASICs doing trillions of hashes per second.
Proof of work vs proof of stake
| Proof of work (Bitcoin) | Proof of stake | |
|---|---|---|
| Who adds blocks | Whoever finds a valid hash | Holders who lock up coins |
| Cost of attacking | Energy and hardware, paid continuously | Buying and staking coins |
| New participants | Anyone with hardware and power | Need coins first |
| Verifying history | Check the work in each header | Need extra trust assumptions for old history |
A quick check you can do
Open any block on MyBitcoinExplorer and look at its hash: it starts with many zeros. Those zeros are the visible proof of work.
Key takeaways
- Proof of work is costly to produce but instant to verify.
- Work accumulates block by block, making old blocks the hardest to change.
- Security depends on honest miners having most of the hash power.
Common questions
Is proof of work a waste of energy?
It's the cost of making history expensive to rewrite without a central authority. Whether that's worth it is debated, but the energy isn't incidental: it's the security.
What is the alternative to proof of work?
Other systems use proof of stake. Bitcoin uses only proof of work.
Who invented proof of work?
The idea goes back to the 1990s, notably Adam Back's Hashcash for fighting email spam. Bitcoin applied it to securing a ledger.
Does proof of work make Bitcoin unhackable?
It makes rewriting history extremely expensive. It doesn't protect wallets or exchanges, which is where most thefts happen.