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 with a high enough fee to incentivise miners to confirm both transactions together.

🤔 But Why Though?

Miners select transactions from the mempool (Bitcoin’s waiting room for unconfirmed transactions) based on feerate — satoshis per unit of transaction weight. Transactions with low feerates wait longer; transactions with high feerates get confirmed quickly. If a transaction is sent with too low a fee during a congested period, it can sit unconfirmed for hours or even days. CPFP works because miners evaluate transaction packages, not just individual transactions. A miner who wants to include a new high-fee transaction (the “child”) must also include its parent — the stuck unconfirmed transaction — because the child spends an output that doesn’t yet exist on the blockchain. If the child’s fee is high enough to make the combined feerate of both transactions attractive, the miner will include both. CPFP is most useful when you receive a payment that the sender underfunded — you can’t modify a transaction someone else sent, but you can spend its output (the Bitcoin coming to you) in a new high-fee transaction, rescuing the stuck payment. It’s the alternative to RBF (Replace-By-Fee — a technique for replacing a stuck outgoing transaction with a higher-fee version of itself) when you don’t control the original transaction.

🌍 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 the package — offering a premium delivery fee — which incentivises the depot to process both the original underpaid package and your courier request together. CPFP is that courier label: your new transaction attached to a stuck incoming payment, offering the miner enough to make it worth processing both.