Hard fork vs soft fork: what's the difference in crypto?

A fork is a change to the rules that a blockchain's nodes use to decide which blocks and transactions are valid. The name comes from what can happen to the chain: if nodes disagree about the rules, the history can split into two branches, like a road dividing. Whether that can happen depends on the type of fork.

What a soft fork is

A soft fork makes the rules stricter. Blocks made under the new rules are still valid to nodes running old software, so those nodes keep following the chain without upgrading. Only the block producers (miners on Bitcoin) need to enforce the new rules, and a soft fork is safe once most of them do.

Bitcoin prefers soft forks because they avoid forcing every user to update at once. Two of its biggest upgrades were soft forks:

• SegWit activated on 24 August 2017 at block 481,824. It moved signature data out of the main part of a transaction, fixed a long-standing bug called transaction malleability and made the Lightning Network practical.
• Taproot activated on 14 November 2021 at block 709,632, adding Schnorr signatures and more private, cheaper complex spending conditions.

What a hard fork is

A hard fork changes the rules in a way that is not backward compatible: blocks valid under the new rules are rejected by old nodes. Every node operator has to upgrade, or they end up following a different chain.

Most hard forks are planned and uncontroversial. Ethereum ships nearly all its upgrades this way: London (August 2021) introduced the EIP-1559 fee burn, the Merge (September 2022) switched the network to proof of stake, and Dencun (March 2024) cut costs for layer 2 networks. Client teams agree on the changes in public calls, test them on testnets such as Sepolia and Hoodi, then set an activation block or epoch. The current example is Glamsterdam, whose Hoodi test was postponed in October 2026 after researchers found a gas-accounting risk.

A contentious hard fork is different: part of the community refuses the new rules, and two coins come out of it.

• Bitcoin Cash split from Bitcoin on 1 August 2017 at block 478,558 to allow much larger blocks (8 MB at first).
• Ethereum Classic is the original Ethereum chain that kept running after the July 2016 hard fork at block 1,920,000, which reversed the DAO hack.

Hard fork vs soft fork side by side

How upgrades get switched on

On Bitcoin, soft forks usually activate through miner signalling: miners mark blocks to show readiness, and the change locks in once a threshold is met within a difficulty period (90% for Taproot's "Speedy Trial"). Node operators can then upgrade to enforce the rules themselves.

On Ethereum, developers on the All Core Devs calls choose which proposals (EIPs) go in, release client software, rehearse on testnets and announce a fixed activation point. If a problem appears, as with Glamsterdam, the date moves.

What a fork means for your coins

For a planned upgrade, holders usually do nothing. Your coins stay where they are, and wallets and exchanges handle the update. Only people running their own nodes, or validators, need to install new software in time. Keep your seed phrase private, though: scammers love "upgrade" emails that ask you to "migrate" or "validate" your wallet.

For a chain split, anyone holding coins on the original chain at the fork block owns the same balance on both chains. Exchanges decide whether to list and credit the new coin. Before moving coins after a split, wait for your wallet or exchange to confirm support, because a transaction on one chain may be replayed on the other if the fork lacks replay protection.