Replace-by-Fee (RBF)

Replace-by-Fee (RBF) is a mempool policy that lets a sender rebroadcast a stuck transaction with a higher fee, replacing the original.

Defined in BIP-125 and shipped in Bitcoin Core 0.12.0, opt-in RBF works like this: the original transaction signals it's replaceable by giving at least one input an nSequence value of 0xfffffffd or lower (at least two below the maximum). If congestion makes your transaction stall, you can construct a new transaction that:

• Spends at least one of the same UTXOs.
• Pays at least as much absolute fee as everything it replaces, plus enough extra to cover its own relay bandwidth.
• Pays a higher fee rate than the original. (This one is Bitcoin Core policy rather than a BIP 125 rule; since Bitcoin Core 31.0 the formal test is that the replacement must leave the mempool's feerate diagram strictly better than before.)

Nodes that accept the replacement drop the old transaction from their mempool and take the new one. Miners, looking for the highest fee rate, are more likely to pick yours up.

What RBF buys you:

• Recovery from fee underestimation. You miscalculated fees during a quiet period and got caught when congestion hit; bump and proceed.
• Faster confirmation when urgency changes. You needed it confirmed cheaply, then suddenly you need it confirmed now.

What it costs:

• Zero-confirmation reliability. A recipient looking at "unconfirmed in the mempool" can no longer assume the transaction won't be replaced. RBF-signaled transactions explicitly admit this. (Transactions that did not signal once had somewhat stronger zero-conf standing, but Bitcoin Core has relayed replacements of unsignaled transactions by default since 28.0 in October 2024, and 29.0 removed the switch; see Full RBF.)

Bitcoin Core's wallet GUI has signaled RBF by default since 0.16.0 (2018). Merchants who once accepted zero-conf payments only from non-signaling transactions lost that protection when full RBF became the default; the safe choice is to wait for the first confirmation. See Fee Bumping for the broader concept (RBF is one method; CPFP is the other).