Mempool
Definition
The mempool is the pool of pending, valid transactions that have been broadcast to a network but not yet included in a block.
When you broadcast a transaction, it does not confirm instantly. It first enters the mempool (short for memory pool), the collection of pending, validly signed transactions that nodes hold while they wait to be included in a block. Validators typically select transactions from this pool, often favouring those offering higher gas priority fees, and include them in the next blocks they produce.
Why it matters
The mempool explains the gap between sending and confirmation. If the network is busy and your fee is low, your transaction may sit in the mempool for a while. It also explains why a low-fee transaction can appear “stuck”: it remains pending, and because of the nonce rule, later transactions from the same account cannot proceed until it clears or is replaced with a higher fee.
Common misunderstanding
People often assume the mempool is private. On public networks it is broadly visible: others can observe pending transactions before they confirm. This visibility enables practices such as front-running, where an observer submits their own transaction with a higher fee to be processed first, and it is one reason large or sensitive interactions can be exploited. A transaction in the mempool is also not final; it has not yet changed on-chain state and can still be dropped or replaced before confirmation. Treating a transaction as settled only after it confirms, rather than when it is broadcast, avoids acting on activity that never finalises. Patience during busy periods is often the only thing required.
Related terms
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