What Is the Bitcoin Mempool?

You hit send on a Bitcoin transaction. It doesn’t land in the blockchain instantly — so where does it go in the meantime? It waits in a kind of digital departure lounge called the mempool.

The mempool, short for memory pool, is the holding area where every Bitcoin node keeps track of transactions that have been broadcast to the network but haven’t yet been included in a confirmed block. When you send Bitcoin, your transaction gets relayed from node to node almost instantly, sitting in each node’s mempool while it waits for a miner to include it in the next block.

Every node maintains its own version of the mempool, and they’re usually very similar to each other but not perfectly identical — a node might see a transaction slightly before or after its neighbors. That’s a normal, harmless quirk of a decentralized network rather than a sign of anything being broken.

Miners don’t process the mempool in the order transactions arrived. Since block space is limited and only a fraction of pending transactions can fit in the next block, miners have an incentive to prioritize whichever transactions pay the highest fee per byte of data. During busy periods, the mempool fills up with a backlog and low-fee transactions can sit for hours or longer.

This is also why fee estimation tools exist in most Bitcoin wallets — they look at the current mempool to estimate what fee is likely to get your transaction confirmed within a target number of blocks, rather than asking you to guess blindly.

A useful mental model: an empty mempool means the network is quiet and even low fees confirm quickly, while a backed-up mempool means fees rise and confirmation times stretch out for anyone unwilling to pay a premium.

Want to understand exactly how those transaction fees get set? Continue learning in the Bitcoin Academy.