← GlossaryMining
What Is Proof of Work? How Mining Secures a Blockchain
Proof of work is a consensus method in which miners repeatedly hash a candidate block until the result falls below a target, so that adding a block requires real computing effort and rewriting history requires redoing that effort.
Key takeaways
- Miners vary a number in the block header and hash it again and again until the hash is below a network-set target.
- The valid chain with the most accumulated work wins, so an attacker must out-compute the honest network to rewrite blocks.
- The work is not the useful part by itself: it is what makes changing past blocks expensive.
- Bitcoin, Litecoin and Dogecoin all use it, with different hash functions and block targets.
How proof of work works
A block is a batch of transactions plus a header. To publish a block, a miner must find a header whose hash is lower than a target the network sets. There is no shortcut to doing this: the miner changes a counter in the header (the nonce) and hashes again, millions or billions of times, until one attempt lands under the target. Checking the answer takes a single hash, so every node can verify a block almost instantly even though finding it was expensive.
Why it secures the chain
Each block commits to the one before it, so changing an old block changes its hash and invalidates every block built on top. To make an alternative history win, an attacker would have to redo the work for that block and all later blocks, and then keep going faster than the honest miners. The Bitcoin paper describes the rule nodes follow: they treat the longest chain, meaning the one with the most proof of work behind it, as the truth. In practice the comparison is by accumulated work, which is why difficulty matters.
What it does not do
Proof of work does not make transactions private, and it does not by itself prevent every attack. If one party controls a majority of the hashing power on a chain, it can rewrite recent blocks, which is why smaller chains are more exposed than large ones. Proof of work also uses real electricity by design: the cost is the security.
Related terms in practice
Difficulty adjusts so blocks keep arriving at a steady pace as hashrate changes. Rewards for finding blocks pay miners for the cost, and pools let small miners share that reward. If you mine Scrypt coins such as Litecoin or Dogecoin, your hardware is doing exactly this work with the Scrypt hash function.
Example
A miner assembles a block, hashes its header with nonce 0, then 1, then 2, and so on. Suppose the network target means roughly one attempt in a trillion succeeds. The miner needs about a trillion tries on average, but anyone else can confirm the winning header with one hash.
Put it into practice
See what proof of work earns on your hardware
Enter your hashrate and power cost in the Scrypt mining calculator to see expected Litecoin and Dogecoin rewards, then send the payouts to a wallet whose keys you hold.
Open the mining calculator Get Scrypt for AndroidFrequently asked questions
Is proof of work the same as mining?
Mining is the activity; proof of work is the rule that defines what a valid block is. Miners perform the work, and nodes verify it.
Why does proof of work use so much energy?
Because the security comes from the cost of the work. If producing a block were cheap, rewriting history would be cheap too.
Which coins use proof of work?
Bitcoin, Litecoin and Dogecoin are well-known examples. Some chains, such as Ethereum since 2022, secure themselves differently, with proof of stake.
Written by the Scrypt Wallet team · Updated 2026-10-02. General education, not financial, tax or legal advice.