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What is the mempool?
The mempool (memory pool) is the waiting area where network nodes hold transactions that have been broadcast but not yet included in a block. Miners or validators pick transactions from it to fill the next block. Each node keeps its own mempool in memory.
What it means for you
A transaction in the mempool is pending, not done: it can wait a long time if its fee is low, and one that is never mined can eventually drop out as nodes clear space. Pending transactions are also public, which is what lets others see and react to them before they confirm.
A common mistake: “Once my transaction shows as pending, it is sure to go through.”
In fact: Pending means nodes are holding it in memory, not that a block will include it. A low-fee transaction can wait a long time, and nodes can drop it to make room; until it is mined, nothing has moved on-chain.
How it works
A broadcast transaction travels peer to peer: a node announces it with an inv message, and a peer that wants it requests it and receives the transaction. Each full node that accepts it holds it in non-persistent memory in its own mempool, so mempools differ slightly from node to node. A node that restarts loses its mempool except its own wallet's transactions, and nodes purge some transactions to make room, which is how a never-mined transaction slowly fades from the network. Transactions in a block that becomes stale can be added back to the mempool.
Source: Bitcoin developer guide: P2P network · checked 4 October 2026
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