A zero confirmation transaction, often shortened to "zero-conf" or "0-conf", is a cryptocurrency payment that has been broadcast to the network but has not yet been included in a block. It sits in the mempool of nodes, waiting for a miner to pick it up. A recipient who treats such a payment as complete gets speed, since there is no wait for a block, but accepts a risk that the payment will be replaced or never confirmed.
The idea matters most on Bitcoin (BTC) and similar proof-of-work chains, where a new block arrives on average every ten minutes. Each later block adds one of the transaction confirmations that make a payment progressively harder to reverse. Zero-conf is the starting point of that scale, and the point at which double spending is easiest.
Zero-conf in plain terms
Imagine a shopkeeper who accepts a customer's word that a bank transfer "has been sent", rather than waiting to see the money arrive. For a cup of coffee, the shopkeeper may decide the risk is worth taking, because most people are honest and waiting would annoy everyone. For a car, nobody would hand over the keys on a promise.
A zero-conf payment is a transfer that everyone can see is on its way but that has not yet been written into the permanent record. Until it is, the sender can still try to send the same coins somewhere else.
Why unconfirmed payments are risky
Until a transaction is in a block, nodes hold it only in memory, and different nodes may see different versions. A sender can broadcast a conflicting transaction that spends the same unspent transaction outputs (UTXOs), and miners decide which one to include, usually the one paying the higher transaction fee. Common risks include:
- A race attack, in which the payer sends the merchant one transaction and miners a conflicting one at almost the same moment.
- A fee replacement, in which the payer rebroadcasts the coins to itself with a higher fee before the original is mined.
- A low-fee payment that sits unconfirmed for hours or days and is eventually dropped from mempools, leaving the merchant unpaid.
Replace-by-fee and the end of "first seen"
For years many Bitcoin nodes followed a "first-seen" policy: they relayed the first version of a transaction and ignored later conflicts, which made zero-conf fairly reliable for small payments. That changed in stages. BIP 125, written by David Harding and Peter Todd and implemented in Bitcoin Core 0.12.0 in 2016, let senders flag a transaction as replaceable, so merchants could at least see which payments were openly reversible. Bitcoin Core 24.0 in 2022 added an option for nodes to accept replacements of any transaction ("full RBF"), and version 28.0, released in October 2024, turned it on by default. Bitcoin developers have long held that unconfirmed transactions offer no guarantee, and since the change the advice is plain: on Bitcoin, zero-conf should be treated as unsafe except for amounts a merchant can afford to lose.
Other networks took different paths. Bitcoin Cash (BCH) removed replace-by-fee and promotes zero-conf for retail payments, with tools that alert merchants to conflicting transactions. Chains with fast block times, such as many proof-of-stake (PoS) networks, make the question less pressing by confirming payments within seconds.
Zero-conf in practice
Merchants and payment processors that accept zero-conf usually limit it to small amounts, check that the transaction pays a reasonable fee, confirm that no conflicting version has appeared and wait a few seconds for it to spread across the network. A payment processor may also absorb the risk on the merchant's behalf. Larger payments, deposits at cryptocurrency exchanges and over-the-counter trades wait for one or more confirmations, often several for big sums.
A sender whose payment is stuck at zero confirmations can speed it up by replacing it with a higher-fee version, where allowed, or through child-pays-for-parent, in which the recipient spends the unconfirmed output with a high fee so miners take both transactions together. Users can follow the status on a block explorer.
Safer alternatives for instant payments
For fast payments, the main Bitcoin alternative to zero-conf is the Lightning Network, a Layer 2 system where payments move through channels secured by on-chain transactions and settle in seconds without waiting for blocks. Custodial wallets and exchanges avoid the issue by moving balances inside their own ledgers, and stablecoin payments on fast chains offer near-immediate payment finality. Each option trades zero-conf's simplicity for different trust assumptions, which is why the right choice depends on the amount, the network and how much risk the recipient is willing to carry.