RBF: the replacement rules
BIP125 sets two fee conditions that nodes enforce: the replacement must pay at least the absolute fee of the transaction it replaces, plus an extra fee for its own size at the incremental relay rate (1 sat/vB by default). Bitcoin Core also requires a higher fee rate than the original. The calculator takes the larger of these minimums and of the fee your target rate needs.
RBF minimum = max(old fee + 1 sat/vB ร new vsize, old rate ร new vsize + 1)
RBF fee = max(RBF minimum, target rate ร new vsize)CPFP: paying through a child
Miners select transactions by the fee rate of a package: a parent and its unconfirmed children together. If you control an output of the stuck transaction โ your change, or the payment when you are the receiver โ you can spend it with a child that pays enough to lift the combined rate to the target.
child fee = target rate ร (parent vsize + child vsize) โ parent fee
package rate = (parent fee + child fee) รท (parent vsize + child vsize)Which one to use
- You sent the transaction and your wallet has a "Bump fee" or "Speed up" button: RBF is usually cheaper, because only one transaction is paid for.
- You are the recipient, or the sender's wallet cannot replace: CPFP from the output you received.
- Payment to an exchange or a swap: RBF keeps the same recipient and amount if your wallet does it correctly, but some services credit only after confirmation, so either way works. Do not change the recipient in a replacement.
- A typical child spending one SegWit input to one output is about 110 vB.
Take the target fee rate from a mempool explorer: the rate for the next block or the next few blocks.
Frequently asked questions
+How much do I need to add to replace a transaction?
At least the original fee plus 1 sat per vB of the new transaction, and a higher fee rate than before. To actually confirm soon, pay the current next-block rate.
+Can I use RBF on any transaction?
Your wallet must support building replacements. Since Bitcoin Core 28 (2024) nodes accept replacements by default even without the opt-in signal, but not every wallet offers the option.
+Who can do CPFP?
Anyone who controls an unspent output of the stuck transaction: the sender through the change output, or the receiver through the payment output.
+What if the transaction is dropped from the mempool?
If it stays unconfirmed long enough many nodes drop it (Bitcoin Core's default expiry is two weeks); the coins then remain where they were and the wallet can rebroadcast or send a new transaction.
Calculations run entirely in your browser; nothing you type is sent to KOT.CASH. Results are estimates for information only and are not financial, tax or legal advice.