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 scheduled event, built into Bitcoin's code, that cuts the new bitcoin paid to miners per block in half. It happens every 210,000 blocks, about every four years; the April 2024 halving at block 840,000 cut it from 6.25 to 3.125 BTC, slowing the rate of new supply.Full definition events, Ordinals activity), it can contain hundreds of thousands 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 ↗ Source 1 in the list belowOpen-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 paying the highest fee rates confirm in the next block (~10 min).
• Transactions paying the lowest fee rates may wait hours or days.

Fee estimates

• "High priority" (next block): the live rate in sats/vByte
• "Medium priority" (~30 min): the live rate in sats/vByte
• "Low priority" (~1 hour): the live rate in 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)A piece of bitcoin you received and have not yet spent, like a separate bill in a wallet. Your balance is the total of your UTXOs. Spending uses up whole UTXOs and creates new ones, usually including change sent back to you, and each UTXO carries its own history, which matters for privacy.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), because Sparrow only offers "Increase Fee" on those transactions by default. The network no longer insists on the signal: Bitcoin Core 28.0 made full-RBF the default, so its nodes relay a higher-fee replacement whether or not the original signaled RBF.

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.