What is the mempool?

You sent Bitcoin and your wallet says “pending”? Your transaction may have already reached the network’s nodes without yet being recorded in a block.

The name comes from “memory pool.” Think of a waiting room: transactions wait there until they’re picked to be recorded on the blockchain. But there’s one difference from a line at the bank: whoever arrived first isn’t necessarily served first.

On Bitcoin, each node checks the transactions it receives and, if they meet its acceptance rules, keeps them in its own mempool and relays them to other nodes. There’s no single, central mempool that everyone sees in exactly the same way. Differences in node connectivity and settings can give a different picture of the transactions that are waiting.

Why it matters

When there are more pending transactions than will fit in the next blocks, competition for space increases. Miners usually prefer transactions that offer a higher fee per unit of space they take up, not necessarily a higher total fee or a larger transfer amount.

That’s why a small transfer can be confirmed before a much larger one. How much Bitcoin you send doesn’t give you priority. The state of the mempool helps wallets estimate appropriate network fees, but an estimated confirmation time isn’t a guarantee.

What to watch out for

A transaction in the mempool hasn’t been confirmed yet. If its fee is low, it may keep waiting or be dropped from some nodes’ memory. Being dropped doesn’t by itself mean lost funds or a final cancellation: the transaction can be rebroadcast.

Depending on the wallet and the transaction, you may be able to bump the fee. We suggest first checking the transaction ID (TXID) on a block explorer, rather than assuming that “sent” means confirmed.