Child-Pays-for-Parent (CPFP)

Child-pays-for-parent is a bitcoin fee-bumping technique in which a stuck, low-fee transaction is accelerated by spending one of its outputs in a new transaction that pays a high fee. Miners evaluating the pair together see a combined fee rate worth mining, and since the child cannot confirm without its parent, collecting the reward requires confirming both.

Why it matters

CPFP is the only fee bump available to a receiver. The sender of a stuck payment can often replace it using replace-by-fee, but the recipient controls only the incoming output, and spending that output with a generous fee is their lever for getting paid sooner. The technique matters most during fee spikes, when transactions sent at yesterday's rates can sit unconfirmed for days, and it is built into Lightning Network channel designs, where anchor outputs exist specifically so either party can CPFP a commitment transaction in an emergency.

How it works

Node software evaluates mempool transactions as packages, ranking each transaction by the total fees of the transaction plus its unconfirmed ancestors divided by their combined size, a figure called the ancestor fee rate. A child paying 20,000 satoshis on top of a parent that paid 1,000 can lift the pair's package rate above the current inclusion threshold. Miners maximizing revenue then select both, and the parent confirms as a side effect of the child's generosity.