What is a mempool?

The mempool is the pool of pending transactions that have been broadcast to a blockchain's network but have not yet been included in a block. Every node maintains its own view of the mempool, prioritizing transactions by fee and rebroadcasting them to peers. When a miner or validator builds the next block, they pick transactions from the mempool — typically the highest-paying ones — and include them. The mempool is short-lived and unglamorous, but most of what is interesting and weird about on-chain behavior — fee markets, MEV, front-running, stuck transactions — happens there.

The life of a transaction

A transaction starts on a user's wallet. The wallet signs it locally and submits it to a node (either their own or an RPC provider like Infura, Alchemy, or QuickNode). The node validates the transaction — checks the signature, confirms the sender has funds and a valid nonce, makes sure the gas price is non-trivial — and accepts it into its local mempool. Then it gossips the transaction to its peers, who do the same.

Within seconds, the transaction has propagated across most of the network. It now sits in tens of thousands of mempools, waiting to be included. Validators (on Ethereum) or miners (on Bitcoin) watch their mempool, select transactions that fit in the next block — usually optimizing for fees — and publish the block. Included transactions disappear from the mempool. Excluded ones stay, hoping for a future block.

A transaction has three possible fates: included (good), replaced (the sender submits a higher-fee version with the same nonce), or dropped (the node evicts it from the mempool because it has been waiting too long or the node is low on memory).

Fee markets and congestion

The mempool is the fee market in action. When demand for block space is high — an NFT mint, a market panic, a DeFi opportunity everyone is chasing — the mempool fills up with transactions bidding against each other. Validators pick the highest-paying ones. Fees climb. Users who set low fees get stuck.

On Ethereum, EIP-1559 (see the gas explainer) smoothed this out somewhat by introducing a protocol-adjusted base fee plus a priority tip. Users set a max fee; as long as the current base plus tip fits under the cap, the transaction is eligible. Overall fee levels now respond more predictably to demand than they did in the pre-1559 days.

On Bitcoin, the fee market is simpler and rougher. There is one fee (satoshis per byte), and it is purely an auction. Mempool.space's visualization shows the Bitcoin mempool as a stack of pending transactions, color-coded by fee rate, and the next few blocks being built from the top of the stack.

The public mempool and MEV

The public mempool is visible to anyone. This is both its original virtue (censorship-resistant broadcast) and its most exploitable weakness. Bots watch the mempool for opportunities: pending trades they can front-run, pending liquidations they can back-run, arbitrage between pools that someone else's transaction would create.

Sandwich attacks are the canonical example. A user broadcasts a large DEX trade with generous slippage tolerance. A searcher bot spots it, inserts a front-run buy ahead of it, lets the user's trade execute at the worse price, then dumps in a back-run sell. The user pays the full slippage and the bot takes the spread. See the MEV explainer for the full taxonomy.

The mempool is the environment in which MEV exists. Private mempools — Flashbots Protect, MEV Blocker, bloXroute's blxrbdn — route transactions directly to block builders without broadcasting publicly. For users this usually means less MEV extraction against them; for validators this means seeing transactions they could not see before, via the MEV-boost market.

Stuck transactions

A transaction can be stuck in the mempool if the gas price the sender set is below what nodes currently require, or if an earlier-nonce transaction from the same sender is pending. Most wallets let users replace a pending transaction by submitting a new one with the same nonce and a higher fee (a "speed up") or by submitting a 0-value self-send with the same nonce and higher fee (a "cancel").

Transactions that sit in the mempool long enough get evicted. The exact timeout varies by node software (typically hours to days). A user who submits a transaction and forgets about it may find it has disappeared from the mempool without ever being included — the equivalent of a failed transaction, except without a failure record on-chain.

Private mempools and the MEV supply chain

On Ethereum post-Merge, the path from wallet to block is more complex than it used to be. A user's transaction can take several routes. The public mempool, as described. Flashbots Protect or MEV Blocker, which route through private relays and block builders. An application's own relay (some DEX frontends integrate directly with builders). Or a MEV-aware aggregator like CoW Swap that batches orders and settles peer-to-peer.

The choice affects several things: how much MEV gets extracted, whether the transaction is visible to other actors before inclusion, and how quickly it lands. For small trades, the public mempool is fine. For large trades on busy pairs, a private route is usually better — often saving more than it costs.

The pre-confirmation horizon

An area of active development is "pre-confirmations" — cryptographic commitments from proposers that a specific transaction will be included in the next block before it actually happens. This is meant to give users certainty faster than waiting for the block itself. Mechanisms from EigenLayer-secured proposers, Based Rollups, and various restaking-anchored commitments are in early deployment as of 2025.

If pre-confirmations become widespread, the mempool's role shifts. Transactions might spend less time in the public mempool and more time in direct proposer commitment paths. The overall architecture of block building would change accordingly.

Why it matters

The mempool is where users actually interact with the fee market, where their transactions live between broadcast and confirmation, and where most of the invisible extraction against them happens. Understanding that you are bidding against bots, that your transaction is visible to anyone watching, and that there are private paths available that route around some of the exposure is the difference between using the public mempool as a default and choosing when to be visible. The mempool is not just a detail of the protocol; it is the environment where the economics of the chain actually play out.