The mempool: the waiting room every transaction passes through

The mempool is the set of valid transactions that have been broadcast to the network but not yet included in a block. Every node keeps its own copy, block producers select from it, and — on most public chains — anyone can watch it. That last property is the one with consequences: your intention becomes public before it becomes final.

What happens when you press send?

Your wallet signs the transaction and broadcasts it to a node, which validates it — signature, balance, nonce, fee — and, if it passes, relays it to peers. Within seconds it is in mempools across the network. It is not on the chain, it has changed nothing, and it can still be replaced or dropped.

A block producer then assembles the next block by selecting from its own mempool, typically choosing the transactions that pay the most for the space they consume. Yours is included when it is worth including.

Why do fees spike?

Block space is fixed and demand is not. When a popular launch or a sharp market move sends thousands of users to transact at once, they bid against each other for inclusion. The price of getting into the next block rises until demand falls back — the same auction dynamic that governs any scarce, time-sensitive resource.

This is why fee estimates are estimates. A wallet suggesting a price is guessing at the next few blocks' competition. Our guide to how gas fees work covers the base fee and tip mechanics that translate this competition into a number.

Why do transactions get stuck?

• The fee is too low for current conditions, so producers keep skipping it.
• An earlier transaction from the same account has not confirmed, and ordering rules block the later one.
• The transaction was dropped from mempools during a purge and never rebroadcast.
• It was broadcast to a poorly connected node and never propagated widely.

Most of these are fixable by replacing the transaction with a higher-fee version, or by cancelling it — the mechanics are covered in our guide to stuck transactions and nonces.

Who can see your pending transaction?

Anyone running a node, and everyone who subscribes to a service that does. Automated searchers watch continuously for opportunities: a large swap that will move a price, a liquidation that can be captured, an arbitrage that opens when your trade lands. They then place their own transactions around yours.

This is the origin of maximal extractable value, and it is a structural feature of a public queue rather than a bug in any one application. Our guide to MEV in plain English covers how the extraction works and what defences exist.

How do private relays change this?

A private relay sends your transaction directly to block producers instead of broadcasting it publicly. It never appears in the public mempool, so searchers cannot trade around it. Several wallets offer this as a setting, and some chains route transactions this way by default.

• Benefit: protection from front-running and sandwich attacks on sizeable trades.
• Trade-off: you rely on the relay operator behaving well and including your transaction.
• Not universal: coverage depends on which producers the relay reaches.
• Not privacy: once included, the transaction is as public as any other.

For most people, most of the time, the public queue is fine. It matters when your transaction is large enough that someone can profit from knowing about it in advance.

How do you read mempool data usefully?

Public dashboards visualise the queue as a set of fee bands, showing how many transactions are waiting at each price and how quickly the backlog is clearing. Two readings are worth taking before any non-urgent transaction.

• The current inclusion price — what the next block is actually accepting, rather than the average of the last hour.
• The shape of the backlog — a deep queue at prices above yours means a long wait, not a slow network.
• The trend — a queue that is draining suggests waiting is cheap; one that is filling suggests acting now or much later.
• Time of day and week, since demand follows human activity and quiet periods are genuinely cheaper.

For anything that is not time-sensitive, a habit of checking before sending and deferring by a few hours is the simplest fee reduction available to an ordinary user — no tooling, no protocol knowledge, just not transacting into a spike.