What is the Bitcoin mempool?

The Bitcoin mempool, short for memory pool, is the waiting room for transactions that have been broadcast but not yet confirmed in a block. When you send bitcoin, your transaction travels to nodes across the network, and each node keeps it in its own mempool until a miner includes it in a block. There is no single global mempool, since every node keeps its own copy based on its settings.

How does the Bitcoin mempool work?

When a node receives a transaction, it checks the transaction against its rules and, if it passes, stores it and passes it on to other nodes. Miners build each new block from the transactions in their mempool. Block space is limited, so they generally choose transactions with the highest fee rate, measured in satoshis per virtual byte (sat/vB).

In Bitcoin Core, the default mempool size is 300 MB. The default minimum relay fee is 0.1 sat/vB, so transactions paying less are not accepted or passed on. When the mempool fills up, the node evicts the lowest fee rate transactions first and temporarily raises the minimum fee it will accept.

Bitcoin Core 31.0, released on April 19, 2026, rebuilt the mempool around "clusters" of related transactions. Transactions are now ranked by the fee rate at which they are expected to be mined, and the default cluster limit is 64 transactions.

Why is my bitcoin transaction stuck?

A transaction usually gets stuck because its fee rate is too low compared with everything else waiting. When many people are transacting at once, the mempool grows and the fee needed to get into the next block rises. Your transaction stays valid, but miners keep picking higher paying ones first. For typical wait times, see how long does a crypto transfer take.

How do you speed up an unconfirmed transaction?

There are two common methods, and whether you can use them depends on your wallet.

Replace by fee (RBF) lets you broadcast a new version of the transaction with a higher fee. Since Bitcoin Core 28.0, full RBF is on by default, and later versions made it unconditional, so a transaction no longer needs to signal in advance that it can be replaced. The replacement must pay more in total fees than the one it replaces.

Child pays for parent (CPFP) works from the receiving side or with your change output. You spend an output of the stuck transaction in a new transaction with a high fee. Miners look at the combined fee rate of the pair, so the generous child can pull the parent into a block.

Do unconfirmed transactions expire?

By default, Bitcoin Core drops a transaction from its mempool after 336 hours, or 14 days, whatever its fee. A transaction that never confirms does not move any coins on the blockchain. Other node software and custom settings can behave differently. For more on what you pay, see what are crypto fees.