Mempool

Mempool, short for "memory pool," is the holding area a blockchain node keeps for transactions that have been broadcast and validated but not yet locked into a block. There is no single global mempool: every node runs its own independent copy, so the exact set of pending transactions a wallet or explorer displays can vary slightly from node to node.

Before a transaction enters the mempool, the receiving node checks that its signature is valid, that the sender controls sufficient funds, and that the inputs are not already spent elsewhere, a safeguard against double spending. Once accepted, the transaction waits until a miner or validator selects it for the next block, generally prioritizing whichever transactions pay the highest fee per byte or per unit of computation.

When network demand outpaces available block space, such as during a sudden surge in trading activity or a popular token launch, the mempool backs up: thousands of transactions compete for a limited number of block slots, pushing users to raise their fees just to move up the queue. On Bitcoin, replace-by-fee lets a sender rebroadcast a stuck transaction with a higher fee, while a recipient can use a child-pays-for-parent transaction to speed up a payment they did not create. Public mempool explorers let anyone inspect pending transactions and gauge current fee levels before submitting a payment.

Because a transaction sitting in the mempool has not yet reached confirmation, it can still be dropped, replaced, or expire if its fee is too low, which is why merchants and exchanges typically wait for one or more confirmations before treating a payment as final.