Level 9 · Advanced technical layer

Mempool, RBF, CPFP and fee bumping

How local mempool policy treats replacements and transaction packages when a payment is waiting.

Article: 92
Reading time: 15 minutes
Reviewed: 26 September 2026

In a nutshell

Every node maintains its own bounded mempool under local policy. RBF replaces an unconfirmed transaction with one paying a sufficient higher fee; CPFP spends an output with a high-fee child so miners may prefer the package.

A child-friendly recap

In very simple terms

Every node has its own waiting list. RBF can replace your payment with a higher-fee version and CPFP attaches a costly child, but neither promises a confirmation time.

There is no single mempool

Nodes can receive transactions in different orders, set different size limits or reject entries under their policy. A transaction missing from one mempool may exist in another; acceptance is not a reservation of block space.

RBF replaces a conflict

A replacement spends at least one of the same inputs. Bitcoin Core policy checks conflicts, fee increase, feerate and resource limits. The original may still be mined by a miner that saw or chose it, so unconfirmed recipients must not treat RBF labels as a guarantee.

CPFP raises package economics

A recipient can spend a parent output with a sufficiently high-fee child. Miners evaluating the ancestor package may include both when their combined feerate is attractive. It requires a spendable output and consumes more block space.

Wallets need a plan

Choose a reasonable initial feerate, preserve change that can be bumped, and understand the wallet's replacement support. Fee estimators use recent history rather than a timetable; neither RBF nor CPFP guarantees a particular confirmation time.

Level 9 · Advanced technical layer

Terms to know

RBF: Replacing an unconfirmed conflicting transaction under mempool policy.
CPFP: A high-fee child used to improve a parent-and-child package's incentive.
Package feerate: Fees divided by size across related transactions considered together.

Common misconception

Once every explorer shows a transaction, it owns a fixed place in the queue.

A more accurate explanation

Mempools are local and change with policy, conflicts, eviction and new blocks; no global queue position exists.

A more accurate explanation

Does RBF enable theft?

An unconfirmed payment is not final with or without RBF. Replacement makes fee adjustment explicit; recipients should match confirmation requirements to value and threat.