Finality in Ethereum

The assurance that a set of transactions on a blockchain is permanently confirmed and cannot be altered without significant economic cost.

What is Finality in Ethereum?

Finality in Ethereum refers to the point at which a block of transactions is considered permanently included in the blockchain, ensuring that those transactions cannot be reversed or modified without incurring substantial economic penalties. In Ethereum’s proof-of-stake (PoS) consensus mechanism, introduced with the Merge in September 2022, finality is achieved through the Beacon Chain’s Casper the Friendly Finality Gadget (FFG). This mechanism ensures that once a block is finalized, altering it would require an attacker to burn a significant amount of staked ETH, making such attacks economically unfeasible.

In Ethereum’s PoS, finality occurs when a block receives enough attestations from validators (at least two-thirds of the total staked ETH) during a process called “justification” and subsequent “finalization” across two epochs (each epoch being 32 blocks, roughly 6.4 minutes). Once finalized, a block is considered immutable unless an attacker controls a massive portion of the network’s staked ETH (potentially billions of dollars’ worth) to execute a 51% attack or attempt to reorganize the chain. This high cost ensures the security and integrity of finalized transactions.

Finality is critical for Ethereum applications, such as decentralized finance (DeFi) and stablecoins, where users need confidence that transactions, like payments or asset transfers, are irreversible. Ethereum’s finality mechanism provides stronger guarantees than probabilistic finality in proof-of-work systems (e.g., Bitcoin), where reversals remain possible with enough computational power, making Ethereum’s PoS finality a key feature for trust and reliability in the ecosystem.