Mempool
By Menno — 13 years in crypto, 3 bear markets survived, zero paid promotions
Last updated: March 2026
The mempool (memory pool) is the waiting area for unconfirmed transactions before they are included in a block. Miners and validators select transactions from the mempool, typically prioritizing those with higher fees.
The mempool is a temporary holding area where every blockchain node stores unconfirmed transactions waiting to be picked up by miners or validators and included in the next block. Each node maintains its own mempool, so there is technically no single universal mempool — rather a distributed set of pending transactions across the network.
When you submit a transaction, it broadcasts to nearby nodes, which verify its validity (correct signature, sufficient balance) and relay it to other nodes. The transaction sits in mempools until a miner or validator includes it in a block. On Bitcoin, miners select transactions primarily by fee rate (satoshis per virtual byte). During congestion, the Bitcoin mempool can swell to 200,000+ pending transactions, and low-fee transactions may wait hours or days. On Ethereum, the mempool is called the transaction pool and validators prioritize by gas price (base fee + priority tip).
The mempool is critically important for understanding transaction confirmation times and fees. Block explorers like mempool.space (Bitcoin) show real-time mempool size and fee estimates. When the mempool is nearly empty, minimum fees get confirmed quickly. When it's congested, users must bid higher to get timely confirmations. The mempool is also where MEV bots and searchers monitor for profitable opportunities — they watch pending transactions and can front-run or sandwich them before block inclusion.
Frequently Asked Questions
Why is my crypto transaction stuck?
Your transaction is likely in the mempool with a fee too low for current demand. On Bitcoin, you can use Replace-By-Fee (RBF) to rebroadcast with a higher fee. On Ethereum, you can submit a new transaction with the same nonce but higher gas. During low-congestion periods, even minimum-fee transactions clear within a few blocks.
Can you cancel a transaction in the mempool?
On Ethereum, yes — by sending a 0-value transaction to yourself with the same nonce and higher gas. On Bitcoin, only if the original transaction was flagged as RBF-enabled. Otherwise, you must wait for it to either confirm or be dropped from mempools after ~72 hours (Bitcoin default).
Related Terms
Gas Fees
Gas fees are transaction costs paid to blockchain validators for processing transactions. Ethereum gas fees fluctuate based on network demand and can range from $0.50 to $100+ during peak congestion.
Blockchain Node
A blockchain node is a computer that participates in a blockchain network by storing a copy of the ledger, validating transactions, and communicating with other nodes. Nodes collectively maintain the network's decentralization and trustlessness.
MEV (Maximal Extractable Value)
MEV (Maximal Extractable Value) is the profit that block producers can extract by reordering, inserting, or censoring transactions within a block. MEV includes front-running, sandwich attacks, and arbitrage, and has generated over $600 million on Ethereum since 2020.
Blockchain
A blockchain is a distributed, append-only database where data is organized into linked blocks and secured by cryptography. Once recorded, transactions cannot be altered — making it a trustless, permanent public ledger.
Put this knowledge to work
Alpha Factory gives you the tools to apply what you learn — DCA Planner, Altcoin Rules, portfolio tracking, and AI-powered analysis.
Start Free Trial