What is Replace-By-Fee (RBF)?
Replace-By-Fee (RBF) is a Bitcoin feature that allows an unconfirmed transaction to be replaced with a new version that pays a higher fee. This is useful when your transaction is stuck in the mempool due to low fees, as it lets you "bump" the fee to get faster confirmation.
RBF was standardized in BIP125 and is now widely supported by Bitcoin wallets and nodes. When a transaction signals RBF, miners will accept a replacement transaction with the same inputs as long as it pays a sufficiently higher fee.
How RBF Signaling Works
RBF is signaled through the nSequence field in transaction inputs. According to BIP125, a transaction is considered replaceable if any of its inputs have an nSequence value less than 0xfffffffe (4294967294).
Sequence Number Values
0xffffffff(4294967295) - Final, RBF disabled0xfffffffe(4294967294) - RBF disabled, locktime enabled0xfffffffdor lower - RBF enabled
Most modern wallets set nSequence to 0xfffffffd by default, enabling RBF for all transactions. This gives users flexibility to bump fees if needed without any downside when fees are sufficient.
When to Use RBF
Fee Bumping
When your transaction is stuck in the mempool during high-fee periods, RBF lets you create a replacement with a higher fee rate to speed up confirmation.
Canceling Transactions
You can effectively cancel an unconfirmed transaction by creating a replacement that sends the funds back to yourself. This requires paying the replacement fee.
Updating Outputs
RBF can be used to modify transaction outputs before confirmation, such as adding or changing recipients (though this requires careful handling).
RBF vs CPFP
There are two main methods for accelerating stuck Bitcoin transactions:
- RBF (Replace-By-Fee) - Replace the original transaction with a higher-fee version. Requires the transaction to signal RBF and requires access to the sender's wallet.
- CPFP (Child Pays for Parent) - Create a new transaction spending an output from the stuck transaction with a high enough fee to incentivize miners to confirm both. Can be done by either sender or recipient.
RBF is generally more efficient as it doesn't require creating an additional transaction, but CPFP works for any transaction regardless of RBF signaling.
Frequently Asked Questions
What is RBF in Bitcoin?
RBF (Replace-By-Fee) is a protocol feature that allows unconfirmed transactions to be replaced with new versions paying higher fees. It's defined in BIP125 and helps users accelerate stuck transactions.
How do I know if my transaction has RBF enabled?
Check the nSequence value of the transaction inputs. If any input has a sequence number less than 0xfffffffe (4294967294), the transaction signals RBF. This tool automatically checks this for you.
Can I enable RBF after sending a transaction?
No. RBF must be signaled in the original transaction. If your transaction doesn't signal RBF, you cannot replace it. Your only option for acceleration would be CPFP.
Why would someone disable RBF?
Some merchants prefer non-RBF transactions for 0-confirmation acceptance, as RBF makes double-spend attempts easier. However, this security benefit is minimal and most modern payment processors don't rely on 0-conf.
Is RBF safe to use?
Yes, RBF is a standard Bitcoin feature. It doesn't introduce any security risks for confirmed transactions. The "replacement" only works for unconfirmed transactions and requires paying a higher fee.
What fee increase is required for RBF?
The replacement transaction must pay a fee that is both higher in absolute terms and has a higher fee rate (sat/vB) than the original. Most nodes require at least a 1 sat/vB increase plus the fee for the new transaction's size.
Can the recipient bump the fee with RBF?
No, only the sender can use RBF because creating a replacement requires signing with the private keys that control the inputs. Recipients can use CPFP instead by spending their unconfirmed output.
Do all Bitcoin wallets support RBF?
Most modern Bitcoin wallets support RBF, including Bitcoin Core, Electrum, BlueWallet, Sparrow, and many others. Some wallets enable RBF by default while others make it an option during transaction creation.
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