Encyclopedia How the network works · Entry 77
Child pays for parent: how a second transaction can lift the first
In this article
At a glance
Key facts
| Fact | Detail | Source |
|---|---|---|
| Dependency | A child spends an output created by its parent. | [1] |
| Authority | You need the keys or other authorization required to spend that output. | [1] |
| Combined rate | For the simplified package example, combined fee rate is total fees divided by total virtual size; node policy can adjust fees and deduplicate existing transactions. | [3][2] |
Why a child helps its parent
Imagine a low-fee payment with an output controlled by its recipient. The recipient can spend that output onward, creating a dependency. A miner cannot include this child as a valid spend without making the parent’s output available.
The sender may instead control a change output and use that. CPFP is therefore about output control, not exclusively about being the sender or recipient.
Calculate the combined rate
Suppose a parent is 200 vbytes and pays 200 satoshis. A child is 100 vbytes. To make the combined 300-vbyte package pay an illustrative 10 sat/vB, total fees must be 3,000 satoshis, so the child contributes 2,800. Setting the child alone to 10 sat/vB would be insufficient.
The arithmetic describes a target package rate, not a promise of relay or inclusion. Actual software has dependency, package and conflict limits, and a wallet may not support this operation.
Check the output before constructing a child
The parent output must be spendable by your wallet. The child’s selected inputs must together fund its outputs and fee; a wallet may add other inputs if needed. Spending an output you do not control is not an option. A watch-only view of a payment cannot supply the necessary signature.
If the parent is replaced, a child referring to the old parent’s transaction ID may no longer be usable. Follow the actual dependency chain in an explorer and the wallet’s current transaction state.
Direct answers
Questions people ask
Is CPFP the same as RBF?
No. CPFP creates a dependent spend; RBF creates a conflicting replacement. They rely on different outputs, permissions and policy conditions.
Inspect the evidence
The answer and key facts have stable claim links. These records retain the scope and qualification when reused.
Child pays for parent, or CPFP, spends an output of an unconfirmed transaction in a new transaction whose fee can raise the combined package fee rate. A miner evaluating the dependent package can collect both fees, but the parent must precede the child, potentially within the same block. A usable output, wallet support and applicable relay policy are necessary.
Scope: Bitcoin · data through 2026-10-02. Verification: verified · 2026-10-02T18:15:23.891Z.
Link to this claimDependency: A child spends an output created by its parent.
Scope: Bitcoin · data through 2026-10-02. Verification: verified · 2026-10-02T18:15:23.891Z.
Link to this claimCombined rate: For the simplified package example, combined fee rate is total fees divided by total virtual size; node policy can adjust fees and deduplicate existing transactions.
Scope: Bitcoin · data through 2026-10-02. Verification: verified · 2026-10-02T18:15:23.891Z.
Link to this claimRevision history
- — Initial Bitcoin encyclopedia entry at this permanent URL.
- — Added reusable claims, explicit source locators, and matching Markdown and JSON. This publishing change does not itself establish factual verification.
Source register
Sources and references
Retrieval dates and locators are recorded individually.- Bitcoin Developer Guide: TransactionsBitcoin developer documentation contributors
UTXO inputs, transaction outputs, change and the difference paid as a fee.
Locator: P2PKH Script Validation; Transaction Fees And Change · Retrieved: 2026-10-02T17:03:41.190ZOpen source - Bitcoin Core package policyBitcoin Core
Package fee accounting and policy for dependent transaction admission.
Locator: Package Mempool Acceptance Rules; Package Fees and Feerate · Version / scope: Bitcoin Core v29.0 · Retrieved: 2026-10-02T17:21:46.897ZOpen source - Segregated Witness consensus layerBitcoin BIPs contributors
Transaction weight and virtual-size definitions in the SegWit specification.
Locator: Block size; Transaction size · Version / scope: BIP-141; immutable revision pinned in source URL · Retrieved: 2026-10-02T17:22:20.465ZOpen source - walletcreatefundedpsbt RPCBitcoin Core
Input selection, fee units, change output, selected inputs and transaction funding.
Locator: Arguments and result fields · Version / scope: Bitcoin Core 29.0.0 RPC · Retrieved: 2026-10-02T17:03:40.827ZOpen source - Opt-in Full Replace-by-FeeBitcoin BIPs contributors
Historical opt-in replacement signaling and replacement policy conditions.
Locator: Summary; Implementation Details · Version / scope: BIP-125; immutable revision pinned in source URL · Retrieved: 2026-10-02T17:22:20.573ZOpen source
Research and drafting use AI assistance. A separate automated review checks claims against primary sources; no external expert or named human review is implied. Publication, substantive editing, source retrieval and verification are recorded separately. This version was independently checked by an automated reviewer on 2 October 2026.
Editorial method and correctionsDegrees of Satoshi editorial project. “Child pays for parent: how a second transaction can lift the first.” Published 2026-10-02; updated 2026-10-02. https://degreesofsatoshi.com/encyclopedia/child-pays-for-parent/