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A zero-confirmation Bitcoin transaction has been broadcast but is not yet included in a block. It is also called an unconfirmed transaction. A wallet or node may show it as pending, but that does not mean the payment is recorded on the blockchain or safe to treat as final.
What does zero confirmation mean in Bitcoin?
Bitcoin transactions spend outputs created by earlier transactions. A transaction has zero confirmations while it is waiting to be included in a block. Once a block containing it is added to the chain, it has one confirmation; each later block adds another.
Nodes hold unconfirmed transactions in their own memory pools, or mempools. A transaction appearing in a wallet or in one node’s mempool means that observer has seen it; it is not a permanent record, and different nodes may have different views. See the Bitcoin Developer Guide’s explanation of the peer-to-peer network.
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Can an unconfirmed Bitcoin transaction be reversed or replaced?
Before confirmation, another transaction can attempt to spend the same input. Only one conflicting spend can be included in the chain. Because transactions travel through an asynchronous network, seeing one version first does not establish that it will be the one miners confirm. Bitcoin Core describes this as a system with no globally consistent “first” in its Opt-in RBF FAQ.
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How replace-by-fee fits in
Opt-in replace-by-fee (RBF) is a policy that can allow a sender to replace a qualifying unconfirmed transaction with one that pays a higher fee. But RBF is not the only source of double-spend risk: Bitcoin Core notes that an unconfirmed transaction may be double-spent with or without RBF. RBF terminology is also covered in the Bitcoin Developer Guide glossary.
What changes when a transaction gets confirmations?
| Payment state | What it establishes | Risk and visibility |
|---|---|---|
| Zero confirmations | Broadcast, but not included in a block. | Still exposed to conflicting spends; a mempool display is a local, temporary view. |
| One confirmation | Included in a block. | More protection than an unconfirmed transaction, though recent blocks can be replaced. |
| Additional confirmations | More blocks have been added after the block containing the transaction. | Replacement becomes increasingly difficult as blocks accumulate, but confirmation is not an absolute guarantee of irreversibility. |
The Bitcoin Developer Guide explains the confirmation progression and cautions that zero-confirmation payments should generally not be trusted without risk analysis: Payment Processing.
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Is it safe to accept a zero-confirmation payment?
There is no single confirmation threshold that suits every payment. A recipient should weigh the amount, how easily the goods or service can be recovered, transaction details, and current network and software conditions. A merchant might accept the risk for a low-value item delivered immediately, while waiting for one or more confirmations may be more prudent before an expensive or irreversible release. These are risk-management choices, not protocol guarantees.
- For a pending payment, describe it as “broadcast,” “seen by this wallet,” or “pending confirmation,” rather than settled or final.
- Do not assume that a transaction’s first appearance in your wallet proves other nodes see the same version or that it will be confirmed.
- Consider what would happen if the payment were not confirmed and the item or service had already been delivered.
The cited guides do not establish a current, general probability of a successful zero-confirmation double-spend, nor do they prescribe one threshold for all recipients. Their historical timing examples should not be read as present-day network benchmarks. The general meaning also does not establish the current policy of any particular wallet, exchange, merchant, or payment processor.
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