The mempool.
Reading on-chain state and rescuing stuck transactions.

Section 1 · What the mempool is

When you broadcast a Bitcoin transaction, it enters the mempool (memory pool): the waiting area for unconfirmed transactions on every node in the network.

Miners select transactions from the mempool to include in blocks. They prioritize by fee rate. Transactions paying more sats per vByte are confirmed first.

The mempool has no fixed size. During high-demand periods (bull markets, halvinghalvingThe event every four years where Bitcoin's new supply per block is cut in half, slowing the rate of new Bitcoin entering circulation.Full definition events, Ordinals activity), it can contain millions of transactions. During low-demand periods, the mempool clears quickly and even very low fees confirm rapidly.

Section 2 · Reading mempool.space

Go to mempool.space verify×DON'T TRUST, VERIFYClaim: mempool.space is the standard public mempool explorer.Verify at: mempool.space ↗Open-source. Source on GitHub. Self-hostable for privacy..

Fee-rate blocks

• The visualization shows pending transactions grouped by fee rate.
• Each colored block represents a future Bitcoin block.
• Transactions at the top (highest fee) confirm in the next block (~10 min).
• Transactions at the bottom may wait hours or days.

Fee estimates

• "High priority" (next block): X sats/vByte
• "Medium priority" (~1 hour): Y sats/vByte
• "Low priority" (~1 day): Z sats/vByte

Transaction size

• Transactions are measured in virtual bytes (vBytes).
• A simple 1-input, 2-output transaction: approximately 140-200 vBytes.
• Adding inputs (spending multiple UTXOsUnspent Transaction Output (UTXO)The fundamental unit of Bitcoin accounting, the "coins" you hold that have been received but not yet spent.Full definition) increases the size and the cost.

Section 3 · RBF (Replace-By-Fee)

If your transaction is stuck (low fee, not confirming), RBF lets you replace it with a higher-fee version.

Requires: the original transaction was broadcast with RBF enabled (most modern wallets do this by default).

In Sparrow Wallet

1. Go to the transaction.
2. Click "Increase Fee."
3. Set the new fee rate.
4. Sign and broadcast.

The new higher-fee transaction replaces the original in the mempool. Miners confirm the higher-fee version.

Section 4 · CPFP (Child-Pays-For-Parent)

If you received a transaction that is stuck, you cannot use RBF (you did not create the original). CPFP lets you create a new transaction (the "child") spending the unconfirmed output, with a high enough fee to incentivize miners to confirm both the parent and child together.

How to think about it

1. Your incoming transaction pays 1 sat/vByte (stuck).
2. You create a spending transaction paying 15 sats/vByte.
3. Miners see: confirming both earns the combined fee. The combined fee rate is above their threshold.
4. Both confirm together.

In Sparrow Wallet

Use the "Child Pays For Parent" feature in the transaction details. Sparrow's documentation walks through the exact UI.

Section 5 · Looking up your transaction

Paste your transaction ID (txid) into mempool.space to see:

• Current confirmation status.
• Number of confirmations (6+ is considered final for large amounts).
• The fee you paid.
• The UTXOs created.
• The UTXOs spent.

Privacy note

Searching mempool.space from your own self-hosted instance is more private than using the public site. The public site logs your IP and the transaction you looked up. Set up self-hosted mempool via Umbrel or Start9. See Run a Node.