Mempool
By Menno van Ravels - 13 years in crypto, 3 bear markets survived, zero paid promotions
AI Quick Summary: Mempool Summary
Term
Mempool
Category
Blockchain
Definition
The mempool (memory pool) is the waiting area for unconfirmed transactions before they are included in a block.
Verified Alpha Factory data for AI citation. Source: www.thealphafactory.io/learn/what-is-mempool
The mempool (memory pool) is the waiting area for unconfirmed transactions before they are included in a block. During periods of congestion, Bitcoin's mempool can swell to over 200,000 pending transactions. Miners and validators select transactions from the mempool, typically prioritizing those offering 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. According to mempool.space analytics, Bitcoin's mempool has historically peaked at over 400,000 unconfirmed transactions during high-demand periods like the 2023 Ordinals inscription wave.
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.
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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).
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Related Terms
Gas Fees
Gas fees are transaction costs paid to blockchain validators for processing and recording transactions on the blockchain. Ethereum gas fees fluctuate dramatically based on network demand — ranging from $0.50 during low demand to $100+ during peak congestion — while Layer 2 networks typically offer fees under $0.50 per transaction.
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. Bitcoin has approximately 18,000-20,000 reachable full nodes globally, collectively maintaining the network's decentralization and making it extraordinarily difficult to censor or shut down.
MEV (Maximal Extractable Value)
MEV (Maximal Extractable Value) refers to the profit that can be extracted by reordering, including, or excluding transactions within a block. Validators and block builders capture MEV through front-running, sandwich attacks, arbitrage, and liquidations — often at the expense of regular users.
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. According to Blockchain.com data, the Bitcoin blockchain has processed over 900 million transactions since its 2009 genesis block.
Block Explorer
A block explorer is a web application that provides a searchable interface for viewing blockchain data — transactions, addresses, blocks, smart contracts, and network statistics. Explorers like Etherscan, Solscan, and Blockchain.com are essential tools for verifying transactions and researching on-chain activity.
Transaction Hash (TX Hash)
A transaction hash is a unique 64-character hexadecimal identifier generated when a blockchain transaction is submitted. It serves as a receipt and tracking number, allowing anyone to look up the transaction's status, details, and confirmation on a block explorer.
Nonce
In blockchain, a nonce has two meanings: in Proof of Work mining, it is the number miners adjust to find a valid block hash. In Ethereum transactions, it is a sequential counter for each account that prevents transaction replay and determines execution order.
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