CPFP (Child Pays for Parent)

CPFP (Child Pays for Parent) speeds up a transaction that is stuck in the mempoolGlossaryMempoolThe buffer in which a node collects valid but still unconfirmed transactions until a miner includes them in a block. There is no central mempool – every node keeps its own.On the learning path: Stage 5 · Step 4 – First withdrawal →In the glossary →× because its fee is too low. The other way to do this is replace-by-feeGlossaryReplace-by-fee (RBF)A method of replacing a still-unconfirmed transaction with a new version that pays a higher fee. This lets you speed up a stuck payment, as long as it isn’t in a block yet.On the learning path: Stage 5 · Step 4 – First withdrawal →In the glossary →×.

The principle

The stuck transaction (the ‘parent’) has outputs, such as the payment to you or the sender’s change. Whoever controls one of them spends it in a new transaction (the ‘child’) with a high fee. The child can only come after the parent in the blockchain. A miner who wants the child’s high fee therefore has to include both – which is why miners assess their fees together.^[1]

Worked example (own calculation)

• Parent: 141 vbytes, 141 sats fee → 1 sat/vB, too little in this example.
• Target for both together: 10 sat/vB. Child: 110 vbytes.
• Together, 251 vbytes × 10 sat/vB = 2,510 sats. So the child pays 2,510 − 141 = 2,369 sats, around 21.5 sat/vB.

The rate you need depends on demand – for the next block, it’s currently around 2 sat/vB.

Who can use CPFP

• Recipient: passes the incoming amount straight on, for example to an address of their own – useful if an exchange withdrawal sent with a tight fee is stuck.
• Sender: uses their change output.

Rule of thumb from mempool.space: if the transaction can be replaced via RBF, replace it; if you control one of its outputs, CPFP helps.^[2] In wallets, the feature is often called ‘Accelerate’ or ‘Speed up’.