Bitcoin Fee Bump Calculator (RBF & CPFP)
Enter your stuck transaction's size and the fee you paid to see the exact fee a Replace-by-Fee or Child-Pays-For-Parent bump needs to reach today's rates.
Updated October 2026 · Reviewed by the PipeFlare team · Educational only, not financial advice
YOU PAID
0.206 sat/vB
target 10 sat/vB
RBF: NEW TOTAL FEE
1,410 sats
+1,381 sats · 10 sat/vB
CPFP: CHILD FEE
2,481 sats
package 10 sat/vB
To reach 10 sat/vB: RBF replacement fee 1,410 sats (+1,381), or a CPFP child paying 2,481 sats.
Loading live sat/vB rates from mempool.space…
RBF replaces your transaction, so your wallet must support it. CPFP needs an output of the stuck transaction that you control (usually your change, or the payment if you are the receiver). The child's size here is a 1-in / 1-out SegWit spend; use your wallet's figure if it shows one.
How Much Fee a Bump Needs
With Replace-by-Fee (RBF), the replacement must pay your target rate for its size and at least the fee you already paid plus 0.1 sat/vB for its size. So a stuck 141 vB transfer that paid 29 sats needs a replacement paying 353 total sats to clear at 2.5 sat/vB. If you use Child-Pays-For-Parent (CPFP) instead, a new 110 vB child transaction must pay 599 sats so that parent and child combined average 2.5 sat/vB.
Miners select transactions by fee rate, measured in satoshis per virtual byte (sat/vB). Transfers with low fee rates wait while miners confirm higher-paying entries first. You can learn more about why fees spike.
RBF costs less than CPFP in this example. In our 141 vB transfer example, RBF requires 324 additional sats over what you already paid. CPFP requires 599 sats because the child must pay for the parent's missing fee and also cover its own 110 vB size.
How to Use the Calculator
Start by entering your existing transaction details in STUCK TRANSACTION SIZE (vB) and FEE YOU PAID (sats). You can locate both numbers inside your wallet transaction details or by searching your transaction identifier on a public block explorer. If you need to estimate your transaction size from scratch, use our bitcoin transaction size calculator.
Next, select your desired fee tier under TARGET RATE. The calculator pulls live rates directly from the mempool.space REST API, including Next block, ~30 minutes, ~1 hour, Economy, and Minimum relay. If you have a specific rate in mind, choose Custom rate and type your figure into CUSTOM RATE (sat/vB).
The advanced parameters populate automatically. REPLACEMENT SIZE (vB, RBF, optional) defaults to the stuck transaction's size. CHILD SIZE (vB, CPFP) defaults to 110 vB, representing a standard 1-input, 1-output native SegWit spend under BIP141. INCREMENTAL RELAY FEE (sat/vB) defaults to 0.1 sat/vB.
Review your results under YOU PAID, RBF: NEW TOTAL FEE, and CPFP: CHILD FEE. The calculator gives sat totals, fiat conversions, and a summary sentence you can copy before updating your wallet. If you plan to make an entirely new transfer later, you can price a new send in the Bitcoin transaction fee calculator.
How the Math Works
The calculator evaluates RBF using node replacement rules. Under BIP125, rule 3 requires the replacement to pay an absolute fee at least as high as the original. Rule 4 requires the replacement to pay for its own size at the node's incremental relay fee:
new total fee = max(target rate × replacement size, fee you paid + incremental relay fee × replacement size)
The result rounds up to the nearest whole satoshi. Bitcoin Core 29.1 lowered the default minimum relay fee and incremental relay fee to 0.1 sat/vB (100 satoshis per kvB).
CPFP calculates the fee needed on a secondary transfer to bring the entire package up to your target rate:
child fee = target rate × (parent size + child size) − fee the parent paid
package rate = (parent fee + child fee) ÷ (parent size + child size)
For our stuck 141 vB transfer paying 29 sats (around 0.21 sat/vB) aiming for 2.5 sat/vB:
RBF check 1: 2.5 × 141 = 352.5 → 353 sats
RBF check 2: 29 + (0.1 × 141) = 43.1 → 44 sats
The larger value is 353 sats. Because you already paid 29 sats, the additional cost to broadcast is 324 sats.
For CPFP with a 110 vB child:
child fee: 2.5 × (141 + 110) − 29 = 627.5 − 29 = 598.5 → 599 sats
Paying 599 sats on the child produces a combined package of 628 sats across 251 vB, yielding an average rate of 2.5 sat/vB.
When the Answer Is Wrong for You
Your wallet and the outputs you control decide which of the two bumps you can use. RBF requires that your wallet application supports fee replacement. If your wallet lacks a fee bump option, you cannot broadcast an RBF replacement.
CPFP requires an unspent output that you control. If you are the sender, you need an unspent change output from the stuck transaction. If you control no output from the transaction, you cannot use CPFP. If neither bump is open to you, our guide to fixing a stuck bitcoin transaction covers waiting it out.
Relay settings can also change your results. Older Bitcoin Core nodes that have not upgraded to version 29.1 enforce an incremental relay fee of 1.0 sat/vB instead of 0.1 sat/vB. If your replacement routes through older nodes, adjust the INCREMENTAL RELAY FEE field to 1.0 sat/vB.
Fee rates can move between checking and broadcasting, so reload this page right before you sign the bump and check that the TARGET RATE tier still shows the rate you used.
Common Questions
How to fix a stuck Bitcoin transaction?
You can speed up a stuck transfer by broadcasting a higher-paying replacement with RBF or by creating a CPFP child transaction. If you need step-by-step wallet instructions, follow our guide to fixing a stuck bitcoin transaction.
Should I use RBF or CPFP?
Use RBF when your wallet supports it because replacing the transaction directly requires fewer total satoshis. Choose CPFP if your wallet lacks RBF support or if you are the recipient waiting on an incoming payment.
Why does RBF need more than just the target rate?
A replacement transaction must pay an absolute fee higher than the original transaction while also paying enough extra fees to cover its own virtual size at the incremental relay fee rate.
What should I do if my Bitcoin transaction is still pending?
Compare the rate you paid (YOU PAID in the calculator above) with today's tiers in the TARGET RATE menu to see how far behind the market your transfer sits. If fee rates have dropped, you can wait for blocks to clear, or you can use our calculator to locate the exact fee required for an RBF or CPFP bump.
Can the receiver speed up a transaction?
Yes, the receiver can speed up an unconfirmed transfer using CPFP. Because the recipient controls the output address receiving the payment, they can spend those unconfirmed funds into a new transaction with a high fee that incentivizes miners to confirm both transfers together.
Sources
General information, not financial advice.