The mempool is where transactions live between the moment you broadcast them and the moment they get included in a block. Every full node in a blockchain network maintains its own copy of the mempool — a pool of pending transactions that have been received from users or peers, verified as valid, and are waiting for a miner or validator to include them in an upcoming block. When a block is produced, the included transactions are removed from the mempool, and the rest wait their turn.
“Mempool” is short for “memory pool” because early clients kept it in RAM. Modern clients usually persist it to disk but the name stuck. Different nodes can have slightly different mempools depending on which transactions they have received, their relay policies, and their local rules for what they consider valid. This means there is no single canonical mempool, just a fuzzy approximation that various public services aggregate and display.
The Gas Market in Practice
During normal network operation, the mempool is where the real-time gas market plays out. Transactions with higher fees get picked up faster because miners and builders prefer to include them first. Transactions with low fees can sit in the mempool for minutes, hours, or longer if the network is busy, and sometimes get dropped entirely if they expire or get replaced by a higher-priced version. This is why wallets display different fee tiers (“slow”, “fast”, “fastest”) — they are estimating what fee level you need to clear the mempool in a given time window.
When the network gets congested, the mempool backs up. During the 2021 NFT boom, the Ethereum mempool was routinely measured in the hundreds of thousands of pending transactions, with fees climbing to hundreds of gwei as users outbid each other for inclusion. The mempool size and fee distribution are the things to watch if you want to understand network conditions — sites like etherscan.io/gastracker and mempool.space (for Bitcoin) show real-time views that are more informative than a single “current gas price” number.
Why the Public Mempool Is a Problem
The public mempool means that anyone running a node can see your transaction before it is confirmed. For a simple transfer, this does not matter. For a DEX swap or a liquidation, it matters enormously. MEV bots watch the mempool for transactions they can profitably exploit — sandwich attacks, arbitrage, front-running, back-running — and they operate at machine speed. The time between your transaction hitting the mempool and a bot constructing a sandwiched pair around it is measured in milliseconds. By the time your transaction lands in a block, the bot has already positioned itself to extract value.
Several solutions exist. Private mempools — Flashbots Protect, MEV Blocker, MEV-Share — let you submit transactions directly to block builders without them going through the public mempool first, which prevents MEV bots from seeing the transaction in time to act on it. Sequencer-based L2s (Arbitrum, Optimism, Base) have private mempools by design, because the sequencer orders transactions centrally before batching them to L1 — this trades decentralisation for MEV protection. Commit-reveal schemes hide transaction content until inclusion is guaranteed, at the cost of additional latency.
For anyone trading non-trivial size through a DEX, skipping the public mempool is now table stakes. MetaMask shipped private-mempool routing as a default for some users in 2023. Rabby and other wallets have it as a prominent option. If you are still submitting large swaps through the public mempool with default settings, you are leaving money on the table, and the money is being taken by bots.
Bitcoin’s Mempool
Bitcoin has its own mempool, which works similarly in principle but is much smaller in volume than Ethereum’s. Bitcoin transactions are simpler (no smart contract calls), the block time is longer (10 minutes instead of 12 seconds), and there is no equivalent of DEX-swap MEV, so Bitcoin mempool dynamics are mostly about fee markets rather than about front-running. The site mempool.space is the best public view, and it shows pending transactions grouped by fee tier, projected confirmation times, and historical fee data. During periods of Ordinals or runes activity, the Bitcoin mempool can back up dramatically and drive fees to hundreds of satoshis per vbyte for short periods, making transfers temporarily expensive in ways Bitcoin used to be immune to.