The mempool is a waiting room for unconfirmed transactions. When you broadcast a transaction, it does not go straight into a block. It sits in the mempool until a miner or validator picks it up. Every node keeps its own mempool, so the contents vary from node to node. But the general picture is the same: thousands of pending transactions, each with a fee attached, competing for limited block space.

Miners sort the mempool by fee rate. Higher fees get included first. If you pay too little, your transaction can sit for hours or get dropped entirely when the mempool clears. During congestion, fees spike and the mempool swells. Bitcoin's mempool has exceeded 100,000 transactions during busy periods. Ethereum's mempool is smaller but more complex because of smart contract interactions. You can check the mempool on a block explorer to see pending transactions and current fee recommendations. If your transaction is stuck, some wallets let you replace it with a higher fee, a feature called replace-by-fee.

Mempool essentials

• Unconfirmed transactions — waiting to be included in a block
• Fee-based ordering — higher fees get priority
• Node-specific — each node has its own mempool
• Replace-by-fee — bump the fee on a stuck transaction

The mempool is a real-time auction for block space. Understanding it helps you avoid overpaying or waiting too long.