CPFP (Child Pays For Parent)

💡 The Plain-English Definition

CPFP — Child Pays For Parent — is a technique for speeding up a stuck Bitcoin transaction by creating a new transaction that spends its unconfirmed output — the incoming coins — with a fee high enough to make miners confirm both transactions together.

🤔 But Why Though?

Miners pick transactions from the mempool — Bitcoin’s waiting room for unconfirmed transactions — by feerate, the satoshis paid per unit of transaction weight. Low-feerate transactions wait longer; high-feerate ones confirm quickly. So a transaction sent with too low a fee during a congested period can sit unconfirmed for hours or days.

CPFP works because miners evaluate transactions in packages, not just one by one. A miner who wants to include a new high-fee transaction — the “child” — has to also include its parent, the stuck transaction, because the child spends an output that doesn’t exist on the blockchain yet. If the child’s fee is high enough to make the combined feerate of the pair attractive, the miner takes both.

It’s most useful when someone pays you and underfunds the transaction. You can’t change a transaction another person sent, but you can spend its output — the bitcoin coming to you — in a new high-fee transaction, dragging the stuck payment along with it. That makes CPFP the counterpart to RBF (Replace-By-Fee, replacing your own stuck outgoing transaction with a higher-fee version) for cases where you don’t control the original.

🌍 The Real-World Analogy

A package is stuck in a depot because the sender didn’t pay enough postage. You, the recipient, attach a courier label to it — offering a premium delivery fee — which gives the depot a reason to process both the underpaid package and your courier request together. CPFP is that courier label: a new transaction attached to a stuck incoming payment, offering the miner enough to make handling both worth it.