What is proof of work?
The guessing race that secures Bitcoin, and why it uses so much electricity.
Proof of work is how Bitcoin decides who adds the next block. Miners race to find a number that gives the block a hash below a target, which takes enormous numbers of guesses. The winner earns new coins and fees, and the cost of all that work is what makes rewriting history impractical.
The guessing race
To add a block, a miner must find a block whose hash starts with enough zeros (technically, is below a target number). There's no shortcut: the only way is to change a value called the nonce and try again, billions of times a second. When someone finds a winning hash, every other node can check it with a single calculation.
You can try a tiny version yourself in the blockchain demo, where "mining" means finding a hash that starts with 00.
Difficulty keeps time
Bitcoin aims for one block every 10 minutes. Every 2,016 blocks (about two weeks) the network checks how fast blocks came and makes the target harder or easier. More miners joining doesn't mean more bitcoins; it just means more competition.
What miners earn
The winning miner gets the block reward, currently 3.125 BTC since the April 2024 halving, plus the fees of every transaction in the block. As the reward keeps halving, fees are expected to become a larger part of miners' income.
Why it's secure
To rewrite a past transaction, an attacker would need to redo that block and every block after it faster than all honest miners combined, which means controlling more than half of the network's computing power, the so-called 51% attack. On Bitcoin that would cost billions in hardware and electricity. Smaller proof-of-work coins with few miners have been attacked this way.
The energy question
Bitcoin mining uses a large amount of electricity, comparable to that of a mid-sized country. Supporters argue it pays for security and increasingly uses surplus or renewable power; critics say the energy cost is too high. Ethereum moved away from proof of work in 2022 partly for this reason.
Frequently asked questions
Can I mine Bitcoin at home?
Technically yes, but it needs specialized machines (ASICs) and cheap electricity to have any chance of profit. Mining on a normal computer won't cover the power bill.
What is a 51% attack?
When one group controls more than half a network's mining power, it can rewrite recent history and double-spend its own coins. It's practical only against small networks.
Which coins use proof of work?
Bitcoin is the biggest. Litecoin, Dogecoin and Bitcoin Cash also use it.
Where to next
Sources
Written for beginners and checked against the sources listed. Last reviewed October 26, 2026. This page is educational and is not financial, tax or legal advice. How we write and check pages.