Proof of work is a rule for deciding who adds the next block to a blockchain: computers compete to produce a hash of the block, a fixed-length digital fingerprint, that is numerically below a target the network sets. Finding one takes an enormous number of guesses, while anyone can check a winner with one calculation, and that imbalance turns electricity into security. Bitcoin, the largest network built this way, is the example below. The crypto hub covers the assets, and how blockchain works explains blocks and confirmations from the beginning.
How does proof of work produce a block?
A miner fills a candidate block with waiting transactions, then hashes its header over and over, changing one field each time, until a result falls below the target.
- Assemble. The miner takes transactions from its mempool, usually the highest fee rates first, plus one that pays the reward to itself.
- Guess. It hashes the 80-byte header twice with SHA-256, changing the nonce each time. The nonce's 4.29 billion values last a 200-terahash machine about 21 millionths of a second, so miners also vary data in the reward transaction.
- Compare. A result counts only if, read as a number, it is no higher than the target. At the difficulty set on October 29, 2026, each attempt had about a 1 in 6.4 × 10²³ chance.
- Collect. Other nodes verify the block with one hash and start building on it, and the miner receives the subsidy, currently 3.125 BTC per block, since block 840,000 on April 20, 2024, plus the block's fees.
Nodes follow the chain with the most accumulated work, not simply the most blocks, so a long branch of easy blocks cannot outrank a shorter branch of hard ones. The reward's dollar value moves with the live bitcoin price.
How does Bitcoin's difficulty adjustment work?
Every 2,016 blocks, each node compares how long those blocks took with two weeks and scales difficulty by the ratio, never by more than a factor of four.
The 20,160 minutes are 2,016 blocks at the 10-minute spacing in Bitcoin Core's consensus parameters. Because the rule only looks backward, blocks run early or late until the next reset, which is why hash rate and difficulty move together without matching day to day.
Worked example: The adjustment at block 973,728
At 19:14 UTC on October 29, 2026, block 973,728 opened a new difficulty period.
- Previous difficulty, set at block 971,712 on October 16: 140,577,680,051,920
- New difficulty: 148,364,561,003,867
- Ratio: 148,364,561,003,867 ÷ 140,577,680,051,920 = 1.0554, a rise of 5.54%
- Implied time for the period: 20,160 ÷ 1.0554 = 19,102 minutes, or about 13.27 days
- Average gap: 19,102 ÷ 2,016 = 9.48 minutes, which the higher difficulty stretches back toward 10 if hash rate holds steady
Adjustments run both ways: 14 of the 26 between November 12, 2025 and October 29, 2026 were cuts. On November 2, with a quarter of the new period done at 10.15 minutes a block, mempool.space projected a change of about −1.7% at block 975,744.
How much electricity does proof of work use?
When checked on November 2, 2026, the Cambridge Bitcoin Electricity Consumption Index estimated Bitcoin's use at a best guess of 174.12 TWh a year, within a range of 89.5 to 334.99 TWh (Cambridge Centre for Alternative Finance, as of Nov 2, 2026).
The index turns hash rate into power using the efficiency of machines that would be profitable at $0.05 per kilowatt-hour: 16.7 joules per terahash in the central case, with bounds of 9.5 and 29.5, which explains the wide range. Consumption follows the value of the rewards rather than the number of payments, so a block with 5 transactions takes as much work as one with 5,000.
Worked example: The electricity behind one block
- Constant demand behind the best guess: 19.86 GW, or 19,860,000 kW
- Per 10-minute block: 19,860,000 kW × 1/6 hour = 3,310,000 kWh, about 3.31 GWh
- The average US household bought 10,791 kWh in 2022, according to the EIA, so one block used about a year's electricity for 307 homes
- Divided among the 615,767 transactions in the 144 blocks to block 974,215, it comes to roughly 774 kWh each, a figure that would halve if blocks held twice as many payments with no change in energy
How does proof of work defend against a 51% attack?
It makes an attack cost roughly what the honest network costs: rewriting recent blocks means out-hashing all other miners combined for as long as the attack lasts.
A majority miner could build a private branch and release it to replace recent blocks, erasing its own payments, a double spend, and holding back other people's. It could not spend coins without their keys or create coins outside the schedule, because full nodes reject such blocks, limits the iShares Bitcoin Trust prospectus spells out among its risk factors. For Bitcoin, matching the network's about 1,046 exahashes a second (EH/s) at 16.7 joules per terahash would take about 17.5 GW of new machines before cooling.
Small proof-of-work coins are easier targets, because the hash rate needed to overpower them can be rented by the hour. Between January 5 and 7, 2019, Coinbase detected 15 deep reorganizations of Ethereum Classic, 12 containing double spends worth 219,500 ETC, about $1.1 million at the time; the deepest discarded 123 blocks. Waiting for more confirmations raises the price of such an attack, because the extra hash rate must be paid for longer.
Where do you meet proof of work?
You meet it on Bitcoin and some older coins, in mining companies' reports, in bitcoin fund prospectuses and in tax rules for mined coins.
- Networks. Bitcoin and Bitcoin Cash use SHA-256; Litecoin and Dogecoin share Scrypt through merged mining (what is Dogecoin); Monero has used the processor-friendly RandomX since November 30, 2019. Ethereum left proof of work in the Merge on September 15, 2022.
- Miners' reports. Listed miners quote fleet hash rate in exahashes and efficiency in joules per terahash, the inputs to the margin that bitcoin halving explained works through.
- Fund documents. The iShares Bitcoin Trust prospectus cites two attacks on Ethereum Classic in August 2020 that reversed transactions worth more than $5.0 million and $1.0 million; bitcoin ETFs vs owning bitcoin compares the two ways to hold the coin.
- Tax rules. In the US, IRS Notice 2014-21 counts mined coins as gross income at fair market value on the day they are received; a tax professional can apply it to a specific operation.
- Block explorers. They show each block's hash, the difficulty and a countdown to the next adjustment, numbers that what is bitcoin explains.
How is proof of work different from proof of stake?
Proof of stake chooses block producers by the coins they lock up rather than by computing power, swapping an electricity bill for capital that can be destroyed.
| Term | What it means | Relation to proof of work |
|---|---|---|
| Proof of stake | Validators lock coins and take turns proposing blocks | The main alternative, used by Ethereum since 2022; see proof of stake |
| Mining | Running proof-of-work hardware for rewards | The activity, not the rule |
| Hash rate | Guesses the whole network makes per second | Measures the work |
| Difficulty | How hard the target is versus the easiest one allowed | Keeps blocks near 10 minutes |
| Proof of history | Solana's original way of ordering events with a chain of hashes | A clock, not a contest; Solana picks block producers by stake (what is Solana) |
| Proof of reserves | An exchange's evidence that it holds customers' coins | Not a consensus method; see proof of reserves |
Bitcoin vs Ethereum compares the largest network on each method.
What do people get wrong about proof of work?
The most common misreading is that miners solve hard equations; in fact they make blind guesses, and the work counts only because it cannot be faked.
- "More miners make payments faster." Only until the next adjustment raises difficulty and restores the 10-minute average.
- "Energy per transaction is what a payment costs." That ratio moves with transaction counts, while consumption follows miner revenue.
- "51% lets an attacker take anyone's coins." It allows reversing the attacker's own payments, not signing for other people's addresses.
- "Difficulty only goes up." It tracks hash rate both ways, and it was lower after October 29, 2026 than after November 12, 2025.
- "Proof of work is the blockchain." A blockchain is the chained data structure; proof of work is one way for strangers to agree on its next entry.