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Agreement (consensus) · 2 min read

What is finality?

By Oliver James Whitmore · Updated 20 October 2026

In one sentence

Finality is the point after which a transaction recorded on a blockchain can no longer be reversed or replaced under the network's rules.

Ethereum checkpoints: once two in a row are justified by two-thirds of stake, the earlier one is final.Checkpoint 1finalisedCheckpoint 2justifiedCheckpoint 3collecting votes≥ 2/3 of stakevotes arrivingReverting a finalised checkpoint would cost at least one-third of all staked deposits
Ethereum checkpoints: once two in a row are justified by two-thirds of stake, the earlier one is final.

How it works

Blockchains reach finality in two broad ways.

Probabilistic finality is how proof of work behaves. A transaction in the newest block could still be displaced if a competing branch overtakes it. Each further block on top makes that less likely, and the chance falls very quickly; section 11 of the Bitcoin whitepaper works through the numbers. Waiting for six confirmations, about an hour, is a common convention for large amounts — a habit, not a protocol rule. In practice, blocks that are more than a few deep on Bitcoin are very rarely replaced.

Economic or deterministic finality is what many proof-of-stake systems add. Ethereum marks the first slot of every epoch as a checkpoint. When validators holding at least two-thirds of all staked units vote for a checkpoint, it becomes justified; when the next checkpoint is justified as well, the earlier one is finalised. Under normal conditions this takes about two epochs, around 13 minutes. Reverting a finalised block would require at least one-third of all staked deposits to be slashed.

Some networks based on classic Byzantine-fault-tolerant protocols finalise each block as soon as two-thirds of validators sign it, often within seconds, at the cost of halting if too many validators are unavailable.

A simple example

A shop that receives a payment can choose how long to wait before handing over the goods. For a small purchase it may accept the transaction once it is in a block; for something large it may wait for more confirmations, or for the block to be finalised. The wait reflects the risk the shop is willing to accept.

Common misconception

Misconception: One confirmation means the payment is permanent.

In fact: One block can still be replaced if a competing branch becomes longer. The risk falls with each confirmation; on networks with checkpoint finality, it disappears only once the block is finalised.

Where you meet it

  • Confirmation counts in wallets and block explorers.
  • "Safe" and "finalized" labels on Ethereum blocks.
  • Bridges and layer-2 systems, which must wait for finality before acting.
  • Payment services deciding how long to wait before treating a payment as settled.

Sources

This card explains a technical concept for general education. It is not a recommendation to acquire, sell or hold any crypto-asset, and it is not financial advice.