# Proof of work

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.

Canonical page: https://mybitcoinexplorer.com/learn/proof-of-work
Topic: Mining
Published: 2026-10-29
Updated: 2026-10-30


## Hard to make, easy to check
A [hash function](https://mybitcoinexplorer.com/learn/hash-functions) 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.

> Diagram: Real SHA-256 attempts: most fail, then one finally starts with enough zeros — 3

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](https://mybitcoinexplorer.com/learn/51-percent-attack).

## Try it yourself
> Interactive exercise: [open the lesson](https://mybitcoinexplorer.com/learn/proof-of-work).

Bitcoin's real target needs a hash starting with about 19 or more hex zeros. That's why miners use [ASICs](https://mybitcoinexplorer.com/learn/asic-miners) 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.

## Related lessons

- [What is Bitcoin mining?](https://mybitcoinexplorer.com/learn/what-is-bitcoin-mining)
- [Target and difficulty](https://mybitcoinexplorer.com/learn/target-and-difficulty)
- [The longest chain (most work) rule](https://mybitcoinexplorer.com/learn/longest-chain-most-work)
- [SHA-256](https://mybitcoinexplorer.com/learn/sha-256)