Finality and Reorgs
Finality is the point after which a transaction cannot realistically be reversed. Before that, a chain reorganisation (reorg) can replace recent blocks with a different version of history. Exchanges wait for several confirmations before crediting deposits for this reason.
Finality is the point where a blockchain transaction can no longer be undone in practice. A reorg is what happens before that point, when the network swaps recent blocks for a competing version. Traders care because exchanges credit deposits only once they consider them final.
- Category
- Cryptocurrency and Blockchain
- Reading time
- 4 min
- Also called
- Transaction finality, Chain reorganization, Reorg
- Updated
Finality and reorgs in plain words
On Ethereum, a finalised block can only be undone after a critical consensus failure in which an attacker loses at least a third of all staked ether.1
Until then, the tip of the chain can still change, and a block can be dropped if the fork-choice rule reorganises it out.2
Bitcoin has no formal finality. Each new block makes a rewrite harder, and the odds of a successful attack shrink exponentially as an attacker falls further behind.3
How a transaction becomes final
Inclusion in a block
The transaction lands in a block at the chain head, which can still change.
Short forks happen
Expect occasional competing blocks at the same height when two miners succeed almost at once.4
Fork choice resolves it
Nodes pick one branch; blocks on the losing branch are orphaned.2
Confirmations add up
Each block added on top means more blocks an attacker would have to redo to remove the transaction.4
Checkpoints on Ethereum
On Ethereum, only checkpoint blocks at epoch boundaries can be justified and finalised.1
Justified, then finalized
A checkpoint is justified when two-thirds of the stake votes for it, and it becomes final once the next checkpoint on top of it is justified too.1
Worked examples
Example 1 — Waiting for confirmations
About 60 minutesA hypothetical exchange credits Bitcoin deposits after 6 confirmations. Blocks arrive about every 10 minutes.
| Wait time | 6 × 10 min = 60 min |
| Blocks to rewrite | 6 blocks on top of the deposit |
The extra blocks make a rewrite far harder, since the attacker's odds fall exponentially with each block.3
Example 2 — Credited before final
2 ETH at riskA hypothetical trader deposits 2 ETH and a venue credits it after 1 block. The block is then reorged out and the deposit does not reappear.
| Credited | 2 ETH |
| After reorg | 0 ETH on chain |
| Venue shortfall | 2 ETH − 0 ETH = 2 ETH |
This is why venues wait: a block can be reorg'd out by the fork choice algorithm before finality.2
Example 3 — Cost of reverting a finalized block
10M ETH at stakeHypothetically, 30 million ETH is staked on a network.
| One-third of stake | 30M ÷ 3 = 10M ETH |
| What an attacker must lose | At least 10M ETH |
Undoing a finalised block would cost an attacker at least a third of all staked ether, which is why it is so expensive.1
Probabilistic vs economic finality
| Aspect | Bitcoin (confirmations) | Ethereum (finalized checkpoints) |
|---|---|---|
| How finality is reached | More blocks on top | Two-thirds stake vote on checkpoints |
| Is it ever absolute | No, probability keeps shrinking | Final unless a third of stake is destroyed |
| Typical user signal | Number of confirmations | Finalized vs head block |
| Reorg risk | Falls with each block | Possible only before finality |
Where finality matters for traders
Deposits and withdrawals
Know when an Exchange will credit funds.
Settlement risk
Treat unfinalized transfers as pending Settlement.
Arbitrage between venues
Fast moves need funds that will not be reversed, which affects Counterparty Risk.
Volatile markets
During Volatility spikes, waiting times can cost an entry price.
Advantages and disadvantages
Advantages
- Gives a clear point after which a transfer is safe.
- Makes large transfers predictable for venues.
- Economic finality puts a price on attacks.
Disadvantages
- Finality takes minutes, not seconds.
- Probabilistic finality is never absolute.
- Different venues use different confirmation rules.
- Finality can stall if enough validators go offline.
Going deeper
Read as far as you need. Each level builds on the one before it.
Final means locked in
Beginner
A finalized block is information the chain is certain about; it cannot be reverted without a critical consensus failure.1
Confirmations as a safety margin
Intermediate
Venues wait for a set number of confirmations before crediting a deposit, because each one adds to the work needed to reverse it.4
When finality stalls
Advanced
Because finality needs a two-thirds vote, someone controlling a third of the stake could stall finality, even while new blocks keep coming.5
Common mistakes
Treating one block as final
A head block can still be reorged out.
Assuming all chains work alike
Bitcoin and Ethereum reach finality differently.
Ignoring venue rules
Each exchange sets its own confirmation count.
Trading unconfirmed funds
Spending money that may disappear in a reorg.
Check your understanding
Test yourself before moving on. Each answer comes with an explanation.
Key points to remember
- Finality means a transaction cannot be practically reversed.
- Reorgs replace recent blocks before finality.
- Bitcoin finality is probabilistic and grows with confirmations.
- Ethereum finalizes checkpoints with a two-thirds stake vote.
- Venues credit deposits only after their own finality rule.
Frequently asked questions
What is a reorg?
How long does Ethereum finality take?
Is a Bitcoin transaction ever fully final?
Further learning
- Settlement
How trades settle in general.
- Exchange
Why exchanges wait for confirmations.
- Counterparty Risk
Risk while funds are pending.
- Volatility
Why waiting time can matter.
Sources
This article is written in our own words. The sources below support the specific facts, rules and figures marked with a number in the text. Links open the original publisher’s page.
- ethereum.org. Gasper (opens in a new tab) Checked .
- ethereum.org. Ethereum Glossary (opens in a new tab) Checked .
- Satoshi Nakamoto. Bitcoin: A Peer-to-Peer Electronic Cash System (opens in a new tab) Checked .
- Bitcoin Project (developer.bitcoin.org). Glossary (opens in a new tab) Checked .
- ethereum.org. Proof-of-stake (PoS) (opens in a new tab) Checked .
Educational content, not financial advice. Examples use hypothetical prices and simplified costs; actual fees, margin rules and regulations vary by broker, market and country. We are not affiliated with any publisher linked above. Last reviewed . Report an error or request a source credit.