Hard fork vs soft fork: what is a fork?
Blockchains change by rewriting their rules. Whether old software can still follow along decides if the network stays one chain or becomes two.
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A fork is a change to the rules a blockchain's software follows. A soft fork is backwards compatible: nodes that do not upgrade still accept the new blocks. A hard fork is not: old nodes reject new blocks, and if both sides keep going, the chain splits.
What is the difference between a hard fork and a soft fork?
Every node checks new blocks against a rulebook. NIST's blockchain overview describes a soft fork as a backwards-compatible change: non-updated nodes can still transact with updated ones. Bitcoin's developer glossary puts it precisely — a soft fork only makes some previously valid blocks or transactions invalid, so old nodes still recognise the new blocks. A hard fork loosens or changes the rules in a way old software cannot follow, causing what Bitcoin's glossary calls a permanent divergence.
| Soft fork | Hard fork | |
|---|---|---|
| Backwards compatible? | Yes | No |
| Non-upgraded nodes | Still accept new blocks | Reject new blocks |
| Can split the chain? | Not by design | Yes, if some participants keep the old rules |
| New coin created? | No | Possibly, if two chains survive |
What happened in Ethereum's DAO fork?
Many hard forks are planned upgrades that the whole network adopts; ethereum.org notes that its forks usually come from Ethereum Improvement Proposals that change the protocol's rules. The famous exception is 2016.
Ethereum.org's fork history also lists routine upgrades, such as London in August 2021, which changed how fees work, and The Merge in September 2022, the switch to proof of stake. Read proof of work vs proof of stake for that change.
Do you get new coins after a fork?
Only if the fork leaves two chains and the new one credits existing holders — often called an airdrop. In the US, the IRS treats new coins received that way as ordinary income at their market value once you control them, while a soft fork, which creates no new coin, produces no income. If a split happens, follow official guidance from your wallet or exchange before moving coins. For the basics of how chains agree on one history, see what is a blockchain.
Questions readers ask
Is every hard fork controversial?
No. Many hard forks are routine upgrades that the whole network adopts, so the old chain simply stops being used. A split happens only when a meaningful group keeps the old rules.
Is a fork the same as forking code on GitHub?
Related idea, different thing. Copying a project's code to start a new network creates a separate blockchain from scratch; a blockchain fork changes the rules of an existing chain with shared history.
Forks are how blockchains upgrade. Soft forks tighten rules and keep everyone on one chain; hard forks change them and can split the network, as Ethereum's DAO fork showed in 2016.
Sources
- National Institute of Standards and Technology, NISTIR 8202: Blockchain Technology Overview (2018)Primary source
- Bitcoin Project (developer.bitcoin.org), Bitcoin developer glossary (2026)Primary source
- ethereum.org, History of Ethereum forks (2026)Primary source
- Internal Revenue Service, Frequently asked questions on virtual currency transactions (2026)Primary source
Educational content only — not financial, investment, legal or tax advice. Crypto-assets are high-risk and you could lose all the money you put in. Rules differ by country; check with your national regulator. See our risk disclosure and editorial policy.
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