Mempool
In simple terms
The mempool is the queue. Your transaction sits there after you send it and before it is picked up, and how long it waits depends mostly on the fee you attached compared with everyone else's.
Definition
The waiting area where broadcast transactions sit before a miner or validator includes them in a block.
In depth
Each node keeps its own mempool of validated but unconfirmed transactions, so there is no single canonical queue. Nodes apply their own acceptance policies, eviction rules and size limits, and relay what they accept. Block producers select from their view, generally maximising fee revenue, which is why a public mempool is also the surface that makes front-running and sandwich attacks possible: a pending transaction is visible to anyone before it executes.
How does Mempool work?
A wallet broadcasts a transaction to a few nodes, which validate it and relay it onward until most of the network holds it. Block producers order candidates by fee and fill the next block. A transaction that is never selected is eventually evicted, at which point it simply disappears and the funds were never spent. Some wallets allow replacing a stuck transaction with a higher-fee version rather than waiting.
An example
During a busy period an illustrative 200,000 transactions are pending. A transfer sent at a below-average fee waits behind them; the same transfer sent hours later, when a few thousand are pending, confirms in the next block for a fraction of the cost.
Figures are illustrative only.
What beginners get wrong
- Sending the same payment a second time because the first is stuck. Both can eventually confirm, and the money is spent twice.
- Treating an evicted transaction as lost funds. Nothing moved, and the balance is intact.
- Ignoring that a pending transaction is public. Anyone watching can see the intent and act before it, which is what MEV extraction relies on.
Related terms
Part of
How does a blockchain transaction actually work? — the subject page for blockchain mechanics, with all 15 of its definitions in one place.
Educational only — not financial advice.