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What is finality?
Finality is the point at which a transaction can no longer be changed or reversed without an attack the network makes extremely costly. On Ethereum, a block is finalized when two-thirds of staked ETH agree on it, usually within 12 to 15 minutes. Bitcoin has no fixed finality point; reversal gets harder with each confirmation.
What it means for you
A transaction that is included but not yet final is in a block that could, in rare cases, still be replaced. When moving large amounts or bridging between chains, the wait for finality is part of why a receiving service may hold funds before crediting them.
A common mistake: “Once a transaction is in a block, it can never change.”
In fact: Before finality, a block can still be replaced if the network settles on a different chain. On Ethereum, finality follows checkpoint votes; on Bitcoin, reversal only becomes less likely with each new block.
How it works
Ethereum proof-of-stake groups 12-second slots into 32-slot epochs, and the first block of each epoch is a checkpoint. Validators vote on pairs of checkpoints; when a pair gets votes representing at least two-thirds of all staked ETH, the newer becomes justified and the older finalized. Reverting a finalized block would cost an attacker at least one-third of all staked ETH. Bitcoin has no such vote: nodes follow the longest proof-of-work chain, and the chance of reversing a block shrinks exponentially as blocks are added on top, without ever reaching a fixed final point.
An example
Say a slot is 12 seconds; an epoch of 32 slots is then 6.4 minutes. Because a block becomes final only after one checkpoint is justified and a later one confirms it, a block typically waits about two epochs, roughly 13 minutes, before it is finalized.
Sources: ethereum.org: Proof-of-stake, bitcoin.org: Some things you need to know, Bitcoin: A Peer-to-Peer Electronic Cash System (Nakamoto) · checked 4 October 2026
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