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Proof of work, explained
Proof of work is how Bitcoin, and blockchains like it, agree on one history without a central party. Miners spend real energy to add each block, so that history is very costly to change. It is the oldest security model in crypto and the one with the longest record.
Facts reviewed in November 2026.
How it works
Each miner collects new transactions into a block and then searches for a number that makes the block's hash fall below a target. There is no shortcut. The only method is to try numbers, and each try uses electricity.
The first miner to find a valid number sends the block to the network. Every other computer can check the answer in an instant. The miner earns new coins and the fees in the block, and the search starts again on top of it.
To rewrite an old block, an attacker must redo the work for that block and for every block after it, faster than all honest miners together. This is why a proof-of-work chain gets harder to change as it grows.
Why people choose it
Open to all
Anyone with hardware and power can mine. No one needs permission, and no one needs to own the coin first.
A real cost
New coins cost energy to make. The money is tied to something outside the network that no one can create for free.
A long record
Bitcoin has run on proof of work since 2009. No other security model has been tested for as long with as much value at stake.
Proof-of-work blockchains compared
| Network | Algorithm | Block time | Smart contracts | Supply |
|---|---|---|---|---|
| Quai | SHA-256, Scrypt, and KawPoW | About 5 seconds | Yes. The EVM runs on the base layer. | Emission slows as the network grows, and SOAP buybacks burn QUAI. 1.9 billion genesis QUAI was burned. |
| Bitcoin | SHA-256 | About 10 minutes | Limited. Bitcoin Script, with no general smart contracts on the base layer. | Fixed cap of 21 million. The block reward halves about every 4 years. |
| Kaspa | kHeavyHash | 10 blocks per second | No EVM on the base layer. EVM contracts run on layer 2 networks. | Maximum of about 28.7 billion KAS. Emission falls on a fixed schedule. |
| Litecoin | Scrypt | About 2.5 minutes | No general smart contracts on the base layer | Fixed cap of 84 million. The block reward halves about every 4 years. |
| Dogecoin | Scrypt | About 1 minute | No smart contracts on the base layer | No cap. A fixed 10,000 DOGE per block, about 5 billion each year. |
| Bitcoin Cash | SHA-256 | About 10 minutes | Limited. Script and CashTokens, with no EVM. | Fixed cap of 21 million. The block reward halves about every 4 years. |
| Monero | RandomX | About 2 minutes | No smart contracts | No cap. A tail emission of 0.6 XMR per block continues for all time. |
Select a network name for a full comparison with Quai. Ethereum used proof of work until September 2022.
What Quai adds to proof of work
- Speed. A block about every 5 seconds, on a hierarchy of chains that miners secure together.
- Smart contracts. The Ethereum Virtual Machine on the base layer, so Solidity contracts run on proof of work.
- Three algorithms. SHA-256, Scrypt, and KawPoW hardware can all mine Quai.
- Revenue from other chains. Through SOAP, Quai miners also mine Bitcoin Cash, Litecoin, Dogecoin, and Ravencoin. Those block rewards buy QUAI on the open market, and that QUAI is burned.
- Workshares. Near-miss mining attempts are included and paid, which gives small miners a steadier income.
- Payments for AI agents. With x402 Work, an agent pays for a web request with a few seconds of proof of work, and it needs no wallet. Read crypto for AI agents.
- Energy-based money. A second token, Qi, whose supply follows mining difficulty and the cost of energy. Read energy based money.
Questions people ask
What is proof of work?
Proof of work is a way for a blockchain to agree on its history without a central party. Miners use computers to solve a hard puzzle. The first to solve it adds the next block and earns a reward. A valid block proves that real energy was spent, which makes the history very costly to rewrite.
Which blockchains use proof of work?
Bitcoin, Litecoin, Dogecoin, Bitcoin Cash, Monero, Kaspa, and Quai all use proof of work. Ethereum used proof of work until September 2022, when it moved to proof of stake.
Can a proof-of-work blockchain run smart contracts?
Yes. Quai runs the Ethereum Virtual Machine on a proof-of-work base layer, so Solidity contracts work on it. Most other proof-of-work networks, such as Bitcoin and Litecoin, do not run general smart contracts on the base layer.
What is the difference between proof of work and proof of stake?
In proof of work, miners spend energy to produce blocks. In proof of stake, validators lock coins to produce blocks. Proof of work ties security to a cost outside the network. Proof of stake ties it to ownership of the network's own coin.
What is merged mining?
Merged mining lets one miner secure more than one blockchain with the same work. Dogecoin has been merge-mined with Litecoin since 2014. Quai uses merged mining inside its own hierarchy of chains, and through SOAP its miners also mine Bitcoin Cash, Litecoin, Dogecoin, and Ravencoin.
Is proof of work slow?
It does not need to be. Bitcoin targets one block every 10 minutes by design. Quai produces a block about every 5 seconds, which is 120 times faster, and it still uses proof of work.