RBF and CPFP calculatorThe exact fee to unstick it, to the satoshi.

Both cards use the same target, the rate you pick at the top. Next block is the lowest rate currently landing in the next block. If you are not in a hurry, the 30 minute or 1 hour targets are usually much cheaper, and the result updates as you switch.

Nodes do not accept every replacement. If they did, anyone could flood the network by rewriting a transaction over and over for free. So a replacement has to clear three bars, and the RBF card checks each of them.

For an ordinary stuck payment the target is the bar that matters and the relay minimum is far below it. The rules start to bite in two cases: when the target is very close to what you already pay, and when the stuck transaction already has children. Replacing a parent throws its children out of the mempool, and the rules make you pay for what they had paid. The card tells you when that happened.

A worked example

A 141 vB payment paid 21 sats, which is 0.15 sat/vB. You want 1.5 sat/vB. The target needs 1.5 × 141 = 212 sats. The relay minimum is 21 + 0.1 × 141 = 36 sats. The larger wins: the replacement pays 212 sats in total and you add 191. At a BTC price of $81,496, that is about 16 cents.

A transaction that spends coins from another unconfirmed transaction cannot be mined before it. Miners therefore look at the whole chain at once and rank it by the combined fee divided by the combined size. That is the number in the result labelled Miners see, and it can be far below the transaction's own rate.

This trips people up constantly. They bump their own payment to a healthy rate, the payment's own number looks fine, and nothing happens, because the parent underneath it is still at the floor dragging the average down. The calculator handles it: when it finds unconfirmed parents, the RBF card prices the replacement to lift the whole chain to your target, and the CPFP card prices a child for the whole chain.

A real case from our mempool accelerator measurements: a 125 vB transaction at the floor rate had thirteen unconfirmed parents, all at the floor, for a chain of 1,638 vB carrying 169 sats. Its own rate looked like every other floor transaction. To lift that chain to 1 sat/vB needed about 1,469 more sats, the same figure the paid accelerator's own price engine calculated before rounding its offer up to 7,000.

Limits on long chains

Bitcoin Core 31 replaced the old ancestor and descendant limits with clusters of at most 64 transactions and 101 kvB. For an ordinary payment none of this matters. For long chains of unconfirmed transactions, a child can be refused for cluster reasons the old rules did not have; the CPFP guide covers what changed.

The child has to pay for itself and for everything still waiting beneath it, so the arithmetic is about the package, not the child.

Notice the child's own rate in that first row: 356 sats over 110 vB is 3.2 sat/vB, more than twice the target. That is correct and expected. A small child is lifting a bigger parent, so its own rate has to sit well above the average it creates. Do not raise it further because the number looks high.

CPFP costs more than RBF for the same target because you also pay for the child's own bytes. In the example the RBF route adds 191 sats against 356 for the child. When you can choose, the RBF card is usually the cheaper one, and the result says which is cheaper for your transaction.

Most wallets ask for a rate, not a total. Enter the rate shown under the RBF total, then check that the new total fee your wallet displays is close to the card. If the wallet rounds rates to whole numbers, round up; the extra is a few dozen satoshis. The RBF guide and the CPFP guide walk through each wallet screen by screen.

Paid accelerators exist for the case where nobody can bump the fee. For every other case, the number on the cards above is the real cost of the fix, paid to the miner, reaching every pool at once.

The full measurements are in the mempool accelerator guide and the paid accelerator reviews. Neither is a bad service; both are for the narrow case where these cards cannot be used.

How this calculator gets its numbers

Everything runs in your browser against mempool.space's public API. The transaction's size and fee come from its transaction endpoint, its unconfirmed parents and children from the CPFP endpoint, and the targets from the precise fee estimates, refreshed every minute. Nothing you paste is sent to us.