Confirmation
A confirmation tells you how deeply a transaction is buried in the blockchain — and so how hard it would be to undo.
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A confirmation is a block on the chain that includes or builds on your transaction. The confirmation score is the number of blocks that would have to be changed to remove it; any score of one or more means confirmed [1]. bitcoin.org suggests six for high-value transfers [2].
Key points
- 10 confirmations: the transaction is only waiting in mempools.
- 21 confirmation: it is in a block. Each new block adds one.
- 3More confirmations make reversal exponentially less likely, but never strictly impossible.
How do confirmations work?#
When a miner includes your transaction in a block, it has one confirmation. Every block added after that one adds another, because changing your transaction would now mean rebuilding all those blocks too. The Bitcoin whitepaper shows that a dishonest sender’s chance of rewriting a payment falls exponentially with each block added after it [3].
| Step | Value |
|---|---|
| Newest height − your block’s height | 105 − 100 = 5 |
| Add 1 for your own block | 5 + 1 = 6 confirmations |
| Average time for 6 blocks at Bitcoin’s 10-minute target | about 60 minutes |
Ten minutes is an average; bitcoin.org notes an individual confirmation can arrive much sooner or much later [2].
Can confirmations go down?#
Occasionally, yes. If two blocks are found at the same time and the network settles on the other branch, the Bitcoin Wiki explains that confirmation counts can change and transactions missing from the new chain become “0/unconfirmed” again [4]. They are usually mined again shortly. Deep confirmations are what make this harmless.
Which words are related?#
- Block height — used to count confirmations.
- Mempool — where transactions wait at zero confirmations.
- How many confirmations? — the full guide with the whitepaper’s probabilities.
Frequently asked questions#
What does “unconfirmed” mean?
The transaction has been broadcast but is not in any block yet. It can still be replaced or dropped, so treat it as pending.
Why do exchanges require several confirmations?
To make sure a deposit cannot be reversed after they credit it. Each service sets its own number.
Do Ethereum transactions get confirmations?
Wallets often count blocks there too, but Ethereum also marks blocks as “finalized” once validators holding two-thirds of staked ether agree [5].
Sources#
Grade A = primary source (regulator, protocol specification, client code, original author). Grade B = expert secondary source used for explanation only.
- Abitcoin.org developer documentation. Bitcoin Developer Glossary, 2026.
- Abitcoin.org. Some things you need to know, 2026.
- ASatoshi Nakamoto. Bitcoin: A Peer-to-Peer Electronic Cash System, 2008.
- BBitcoin Wiki. Vocabulary (Memory pool, Confirmation, Hash function, Node), 2026.
- Aethereum.org. Gasper, 2026.