Replace-By-Fee (RBF)

Replace-By-Fee, or RBF, is a node mempool policy that allows an unconfirmed transaction to be swapped out for a new version that spends at least one of the same inputs and pays a higher fee. It gives senders a way to speed up a stuck transaction when the network is congested and they originally attached too low a fee.

Opt-in signaling under BIP125

The widely used opt-in form of RBF was formalized by Peter Todd in 2015 as BIP125. A transaction signals that it is replaceable by setting a specific value in the input's sequence field. Wallets that see this flag know a higher-fee replacement may legitimately appear before confirmation. For a replacement to be accepted, BIP125 requires it to pay both a higher feerate and a higher absolute total fee than the transaction it displaces.

Why it matters for users and merchants

For the sender, RBF is the cleanest tool to escape a fee that was too low during a mempool spike. For anyone accepting payments, an RBF-signaled transaction is a clear reminder not to treat zero-confirmation receipts as final, because the sender can still revise the transaction. Some nodes also run full-RBF, which permits replacing any unconfirmed transaction regardless of signaling.

RBF is one of two main fee-bumping strategies. The other is Child-Pays-For-Parent (CPFP), which a recipient can use without the sender's cooperation. Replaceability can also be flagged when broadcasting a finalized PSBT.