Imagine receiving bitcoin but the transaction is stuck in the mempool.
The sender used a fee that's too low, and miners aren't picking it up.

What can you do? This is where Child Pays For Parent (CPFP) comes in.

What is CPFP?

CPFP is a fee-bumping strategy where the recipient of an unconfirmed transaction creates a new transaction (child) that spends the pending funds and includes a high enough fee to incentivize miners to confirm both transactions together.

How Does CPFP Work?

Let's break it down with an example:

1. Alice sends 1 BTC to Bob with a very low fee
2. The transaction gets stuck in the mempool because miners ignore low-fee transactions
3. Bob creates a new transaction (child) spending the unconfirmed bitcoin
4. Bob attaches a high fee to the child transaction
5. Miners see they can collect both fees by including both transactions
6. Both transactions get confirmed in the next block

Technical Details

CPFP works because of two key Bitcoin concepts:

1. UTXO Model: Bitcoin allows spending unconfirmed outputs, enabling the creation of child transactions
2. Mempool Package Evaluation: Miners evaluate related transactions as a package, considering their combined size and fees

Fee Calculation

The effective fee rate for a CPFP package is:

CPFP is particularly useful when:

• You receive a transaction with too low fees
• The sender didn't enable RBF (Replace-By-Fee)
• You need urgent confirmation of incoming funds

Limitations

While powerful, CPFP has some constraints:

• Only the recipient can initiate CPFP
• Requires enough funds to cover the new transaction fee
• Some wallets don't support CPFP natively
• May not work during extreme network congestion