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The Mempool

Bitcoin's waiting room — where transactions queue before being mined

What is the Mempool?

The mempool (memory pool) is where unconfirmed transactions wait to be included in a block. Every Bitcoin node maintains its own mempool.

🚏 Think: A Bus Station

Imagine a bus station where buses (blocks) arrive every 10 minutes. Each bus has limited seats (~4,000 transactions). Passengers (transactions) wait in line, and those who tip the driver the most get priority boarding.

Key point: There's no single "the mempool" — each node has its own copy. They're usually similar but can differ slightly based on when transactions were received.

Mempool Visualization

2,847
Pending TXs
1.2 MB
Mempool Size
45
Next Block (sat/vB)

Transactions by Fee Rate

523 TXs
100+ sat/vB
~Next block
892 TXs
50-100 sat/vB
~1-2 blocks
1,245 TXs
10-50 sat/vB
~3-6 blocks
187 TXs
1-10 sat/vB
Hours/Days/Never

Transaction Lifecycle

  1. Broadcast: You sign and broadcast your transaction
  2. Propagation: Nodes relay it across the network
  3. Mempool: Each node adds it to their mempool
  4. Selection: Miners pick high-fee transactions for their block
  5. Confirmation: A miner finds a block containing your TX
  6. Removal: All nodes remove confirmed TXs from their mempools

What Happens When You're Stuck?

RBF: Replace-By-Fee

If your transaction is stuck, you can broadcast a replacement with a higher fee. The original transaction is dropped from mempools. Most wallets support this if you enable RBF when sending.

CPFP: Child-Pays-For-Parent

If you're the recipient, you can spend the unconfirmed output with a high fee. Miners are incentivized to confirm both transactions together to collect the child's fee.

Waiting it Out

Eventually, mempools clear. Transactions are typically dropped after 2 weeks if not confirmed. The funds return to being spendable as if the transaction never happened.

Mempool Monitoring Tools

Pro tip: Check mempool.space before sending a transaction. It shows real-time fee rates and estimated wait times.