A high fee on a new Bitcoin transaction may still be too low to rescue the payment it spends. With child pays for parent (CPFP), miners weigh the child together with its unconfirmed parent. The number to calculate is the combined fee per virtual byte, rather than the child’s fee alone.
| What you need to know |
|---|
| CPFP spends an output from an unconfirmed parent transaction. |
| A recipient can use it; a sender needs an output they control, such as change. |
| The package fee rate includes both transactions' fees and virtual sizes. |
| A fee increase offers an incentive, not a guaranteed confirmation time. |
Bitcoin Optech’s CPFP explanation describes the child as a higher-fee transaction spending an output from a low-fee parent. The dependency matters: the child cannot confirm before the parent. A miner who includes the child must also include that parent, so the child can make the pair more attractive.
Count the parent’s size as well
Consider a hypothetical parent of 200 virtual bytes with a fee of 100 satoshis. A child of 100 virtual bytes paying 2,000 satoshis has its own fee rate of 20 sat/vB. Together, however, the transactions pay 2,100 satoshis over 300 virtual bytes: just 7 sat/vB. Those are illustrative figures, not a reading of current network demand.
For a hypothetical combined target of 10 sat/vB, the same 300-vB package would need 3,000 satoshis in total fees. Subtract the parent’s existing 100 satoshis and the child must supply 2,900 satoshis. Its individual rate would then be 29 sat/vB. This calculation explains why copying a target package rate directly into the child’s fee setting can undershoot.
The example assumes exactly two transactions. If the parent itself depends on another unconfirmed transaction, there is more size and fee to account for. Use the wallet’s actual transaction estimate; adding inputs or changing outputs can change the child’s size. Available spendable value must also cover its fee.
The output determines who can act
Electrum’s official FAQ says a recipient can create a CPFP transaction, while a sender can do so if the parent has a change output. That makes control of an output the first check. Receiving a payment through an exchange does not by itself give you the ability to sign a transaction spending that exchange’s output.
Electrum documents a “Child pays for parent” option on the unconfirmed transaction in its history. Review the proposed fee before signing and broadcasting. Its separate “Increase Fee” option replaces a transaction the wallet can replace, rather than adding a child. These are different ways to raise the incentive; a receiving wallet cannot simply replace the sender’s payment.
Neither calculation reserves a place in a block. Miner selection, relay policies and competing transactions still affect confirmation. If the output is outside your control or the proposed fee would consume too much of it, the arithmetic identifies a limit, not a reason to send another unrelated payment.
