Replace-By-Fee (RBF)

Replace-By-Fee (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 the fee they originally attached turned out to be too low. Crucially, RBF is a policy, not a consensus rule: nothing in Bitcoin's consensus layer prevents conflicting unconfirmed transactions from existing, and miners have always been free to include whichever version pays them best. RBF simply standardizes how relaying nodes handle the replacement.

Opt-in signaling under BIP125

The widely deployed 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 an input's sequence field (any value below 0xfffffffe). Wallets that see this flag know a higher-fee replacement may legitimately appear before confirmation. For a replacement to be accepted under the standard rules, it must pay both a higher feerate and a higher absolute total fee than the transaction it displaces — the extra fee has to cover the cost of relaying the replacement, so you cannot spam the network with endless free replacements. Many nodes today also run full-RBF, a stricter-free variant that permits replacing any unconfirmed transaction regardless of whether it signaled, which has pushed the ecosystem toward treating every unconfirmed transaction as replaceable in practice.

Why it matters for users and merchants

For the sender, RBF is the cleanest tool for escaping a fee that was too low during a mempool spike: the wallet rebuilds the transaction with a higher fee rate, re-signs it, and broadcasts the replacement. Most modern wallets expose this as a simple "bump fee" button. For anyone accepting payments, an unconfirmed transaction — RBF-signaled or not — is a reminder that zero-confirmation receipts are not final. The sender can revise the transaction right up until it is mined, including redirecting the funds back to themselves. Anything of value should wait for at least one confirmation.

The miner's perspective

From a miner's chair, RBF is straightforwardly rational: block space is scarce, and when two conflicting transactions compete for it, the one paying the higher fee wins a slot in the block template. Fee-bumping traffic is part of what makes a healthy fee market, which matters more to mining revenue with every halving as the subsidy shrinks and fees carry a growing share of the block reward. A home miner or node runner watching their mempool during a fee spike will see waves of replacements as wallets race to stay above the going rate.

Practical habits

The working rules are simple. When sending, let your wallet signal RBF by default — it costs nothing and preserves the option to bump; the only common reason to disable it is a recipient that treats signaled transactions differently. Bump early rather than late: a transaction sitting far below the going rate can linger until the mempool purges it, and a timely bump costs less than a panicked one. Be aware that replacement is not always frictionless — the rules require the new transaction to pay for the block space of what it displaces, and in adversarial corners of the protocol (multi-party contracts, unconfirmed chains of transactions) those same rules can be exploited to make honest replacement expensive, a family of problems known as transaction pinning. For everyday wallet use, though, RBF behaves exactly as intended: a fee you can revise until the network accepts your bid.

RBF is one of two main fee-bumping strategies. The other is Child-Pays-For-Parent (CPFP), which the recipient can use without the sender's cooperation by spending the unconfirmed output with a generous fee. Replaceability can also be flagged when finalizing and broadcasting a PSBT, which is worth remembering when coordinating multi-party transactions.