Glossary

Mempool

Plain-language definition Crypto glossary

Key takeaways

The mempool (memory pool) is the holding area for transactions that have been broadcast to a network but not yet confirmed in a block, with each node keeping its own view.
A broadcast transaction waits in the mempool until a miner or validator selects it for the next block, and because block space is limited, higher-fee transactions are generally picked first.
The size of the mempool is a real-time measure of congestion: a crowded mempool means longer waits and higher fees, while an empty one means fast, cheap confirmations.

Definition

The mempool (memory pool) is the holding area for transactions that have been broadcast to a network but not yet confirmed in a block. Each node keeps its own view of pending transactions.

How it works

When you send a transaction, it spreads across the network and waits in the mempool until a miner or validator selects it for the next block. Because block space is limited, those offering higher fees are generally picked first. Once a transaction is included in a confirmed block, it leaves the mempool.

Why it matters

The size of the mempool is a real-time measure of congestion: a crowded mempool means longer waits and higher fees, while an empty one means transactions confirm quickly and cheaply. Watching it helps users decide what fee to attach.

Example

During a surge in activity, thousands of transactions can pile up in the mempool, pushing fees higher.

FAQ

Frequently asked questions

What happens to a transaction in the mempool?

After you broadcast a transaction, it spreads across the network and waits in the mempool until a miner or validator picks it for the next block. Once it is included in a confirmed block, it leaves the mempool.

Why do higher fees get transactions confirmed faster?

Block space is limited, so miners and validators generally select the transactions offering higher fees first. When the mempool is crowded, attaching a higher fee improves the chance your transaction is picked sooner.

How does the mempool relate to network fees?

The size of the mempool is a real-time gauge of congestion. A crowded mempool means longer waits and higher fees, while an emptier one means transactions confirm quickly and cheaply, which helps users decide what fee to attach.