A hard fork is a change to the rules of a blockchain that is not backward compatible: blocks produced under the new rules are rejected by software still running the old rules. Every node operator has to upgrade to keep following the network. If some refuse, the chain splits in two, and each branch carries on with its own history, its own coin and its own community.
Hard forks are one of the two ways public networks change their protocol; the other is the soft fork, which tightens rules so that old nodes still accept the new blocks. The general idea of a chain diverging is covered in the Fork entry. Hard forks are used both for planned, uncontested upgrades, as Ethereum does regularly, and for contested splits such as Ethereum Classic in 2016 and Bitcoin Cash in 2017.
Hard fork in plain terms
Imagine a board game club whose rulebook says a player may hold at most seven cards. One evening most members agree that, from now on, ten cards are allowed. Members who keep the old rulebook look at a hand of ten cards and declare it cheating. If they never change their mind, the club breaks into two tables playing slightly different games.
A hard fork is that kind of rule change. Loosening or rewriting the rules means old players cannot recognise new moves as legal. When everyone switches together, the club simply carries on with the new rulebook. When a group stays behind, there are now two clubs, each convinced its own table is the real one.
How a hard fork works
Every full node checks each new block against a fixed set of consensus rules: maximum block size, valid signatures, how new coins may be created and so on. A hard fork changes those rules so that something the old software calls invalid becomes valid. Developers usually schedule it for a specific block height or timestamp, publish new client software in advance and ask exchanges, wallets, miners and validators to upgrade before that point.
At the activation height, upgraded nodes begin accepting blocks under the new rules. If nearly all participants have upgraded, the old chain simply dies off and nobody notices a split. If a meaningful share of miners or users keeps the old software, two chains with a shared history up to the fork point continue side by side. Anyone who held coins before the split then holds an equal balance on both chains, because both inherit the same ledger of unspent outputs or account balances.
Notable hard forks
- Ethereum and Ethereum Classic (2016). After the attack on The DAO, a DAO that held a large share of all ether, Ethereum developers activated a hard fork at block 1,920,000 on 20 July 2016 that moved the drained funds to a recovery contract. A minority rejected the intervention and kept the original chain, which became Ethereum Classic (ETC).
- Bitcoin Cash (2017). On 1 August 2017, after years of dispute over block size, a group of miners and developers split from Bitcoin at block 478,558 to create Bitcoin Cash (BCH) with larger blocks. Bitcoin Cash itself later split again in November 2018, producing Bitcoin SV (BSV).
- Bitcoin Gold (2017). Bitcoin Gold (BTG) forked from Bitcoin in October 2017 to change the mining algorithm, aiming to reduce the advantage of specialised mining hardware.
- Planned Ethereum upgrades. Most Ethereum network upgrades, from Spurious Dragon in 2016 to Fusaka on 3 December 2025, have been uncontested hard forks bundling several Ethereum Improvement Proposals. The 2022 move from proof of work to proof of stake, known as the Merge, was another.
Risks and safeguards
The main danger in a contested hard fork is the replay attack. Because both chains share the same addresses and transaction format, a payment signed for one chain may also be valid on the other, letting someone rebroadcast it and move the victim's coins on both. Well-designed forks add automatic replay protection so that a transaction can only be valid on one branch.
Other risks are practical. Exchanges may freeze deposits and withdrawals around the fork height, wallets may show the wrong balance, and confirmations become unreliable while the network sorts itself out. A minority chain with little mining power can also be cheap to attack, as several smaller proof-of-work forks have learned through a 51% attack.
Governance and criticism
Because a hard fork needs almost everyone to move at once, it is as much a social event as a technical one. Bitcoin's community, whose upgrade process runs through Bitcoin Improvement Proposals, strongly prefers soft forks and has not made a planned hard fork in its modern history; supporters see this as a guarantee that the rules, including the 21 million coin limit, cannot easily be rewritten. Ethereum takes the opposite view and uses scheduled hard forks as its normal upgrade path.
Critics argue that contested forks dilute a network's brand, confuse users and hand coin holders an "airdrop" whose value comes from attention rather than technology. Supporters reply that the ability to fork is the ultimate check on developers and miners: anyone unhappy with a project's governance can take the code, which is open source, and leave.