Skip to main content

Child Pays for Parent (CPFP) | Bitcoin Glossary | Mapping Bitcoin

Child Pays for Parent (CPFP)

Protocolo

Also known as: CPFP, ancestor fee bumping

A fee-bumping technique where a new transaction (child) spends an output of an unconfirmed transaction (parent) with a high enough fee to incentivize miners to confirm both transactions together. CPFP allows recipients to accelerate incoming payments.

Overview

Child Pays for Parent (CPFP) is a fee-bumping mechanism that allows a recipient (or sender) to accelerate the confirmation of a stuck, low-fee transaction. By creating a new "child" transaction that spends an output of the unconfirmed "parent" transaction and attaching a high fee, the combined fee rate of both transactions becomes attractive enough for miners to include them together in a block.

How CPFP Works

Scenario: Parent TX has a low fee and is stuck in the mempool

Parent TX (low fee: 1 sat/vB)
  Input:  0.5 BTC
  Output: 0.3 BTC → Bob
          0.199 BTC → Alice (change)
  Fee:    0.001 BTC

Bob creates a child TX spending his output with a high fee:

Child TX (high fee: 50 sat/vB)
  Input:  0.3 BTC (from parent's output to Bob)
  Output: 0.28 BTC → Bob's new address
  Fee:    0.02 BTC

Miner's calculation:
  Combined package size: ~400 vB
  Combined fees: 0.001 + 0.02 = 0.021 BTC
  Effective fee rate: ~52.5 sat/vB  ← attractive to mine!

CPFP vs Replace-by-Fee (RBF)

Both CPFP and RBF are techniques for accelerating unconfirmed transactions, but they work differently:

  • CPFP: Creates a new transaction; can be done by the recipient or sender
  • RBF: Replaces the original transaction; can only be done by the sender

CPFP is particularly valuable because it gives recipients the ability to accelerate payments they are waiting to receive, without requiring any cooperation from the sender.

Miner Behavior

Rational miners evaluate transactions not individually but as packages. When a high-fee child transaction depends on an unconfirmed parent, the miner must include both to collect the child's fee. Bitcoin Core's mining algorithm considers these ancestor packages when selecting transactions for block templates.

Common Misconceptions

A common misunderstanding is that CPFP is free. In reality, the child transaction must pay enough fees to make the combined package attractive, which means the child's fee effectively subsidizes the parent's low fee. The total cost can be significant during periods of high fee pressure.

Termos Relacionados

Replace-by-Fee (RBF)Protocolo

A mechanism (BIP125) that allows a sender to replace an unconfirmed transaction in the mempool with a new version that pays a higher fee. RBF is useful for fee bumping transactions that are stuck due to an initially low fee rate.

FeeEconomia

The amount of bitcoin a sender pays to miners to incentivize the inclusion of their transaction in a block. Fees are calculated as the difference between total inputs and total outputs, and are typically expressed in satoshis per virtual byte (sat/vB).

MempoolProtocolo

The memory pool where each Bitcoin node stores valid but unconfirmed transactions waiting to be included in a block. Miners select transactions from the mempool based on fee rate, prioritizing higher-paying transactions when block space is scarce.

Unconfirmed TransactionProtocolo

A Bitcoin transaction that has been broadcast to the network and is sitting in the mempool but has not yet been included in a mined block. Unconfirmed transactions have no guarantee of being finalized and can potentially be replaced or dropped.