Understanding Mempool in Blockchain

What is a mempool in blockchain, and how does it work? This short article will explain everything, including mempool.space.
Written by
Shivam Srivastava
June 27, 2024
min. read


Mempool, short for "memory pool," is a cornerstone in understanding the lifecycle of blockchain transactions. It transcends technical jargon, serving as a pivotal element that significantly influences the efficiency, speed, and reliability of transaction processing across diverse blockchains.

In this article, you will find out what a mempool is and why chains cannot live without it. We will dig into its mechanics, its critical role in transaction validation, and how it helps maintain the inner workings of blockchain networks. We will also check out the intricacies of tools like mempool.space and examine how the mempool count – a key metric in blockchain operations – impacts overall transaction processing.

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What is Mempool?

Definition and Basic Functionality

Mempool refers to a temporary storage area for transactions that are waiting to be confirmed and added to a block. When a transaction is initiated on a blockchain network, it does not immediately get added to the blockchain. Instead, it first enters the mempool, where it awaits. Imagine it as a swimming pool that validators continuously empty while users keep adding to it.

Blockchain Mempool Functionality:

  1. Transaction Validation and Verification: Before entering the mempool, transactions undergo preliminary approval. Nodes check if the transactions comply with the network's rules, such as verifying digital signatures and ensuring that the sender has sufficient funds. This initial check prevents invalid transactions from clogging the mempool.
  2. Prioritization and Fee Market: The mempool essentially determines the cost of individual transactions. These transactions typically include a fee, which serves as a reward for miners or validators processing them. Transactions with higher fees are prioritized because miners tend to select them first for the greater reward, thereby creating a competitive market.
  3. Handling Network Congestion: During times of high transaction volume, the mempool can become congested with transactions waiting to be included in a block. This congestion can lead to increased transaction fees and longer processing times. Hence, if you want to pick ideal times for cheap transactions, you need to watch the mempool, for instance, using the Tatum mempool tracker extension. You will find more tools on our GitHub page.
  4. Transaction Broadcasting and Propagation: The mempool also facilitates the broadcasting and propagation of transactions across the network. When a node receives a new transaction, it adds it to its mempool and broadcasts it to other nodes. This propagation continues until the transaction reaches a significant portion of the network.
  5. Blockchain Network Health Indicator: Analyzing the mempool provides insights into the overall health and efficiency of a blockchain network. A perpetually overloaded mempool might indicate scalability issues, while a consistently low mempool count could suggest low network usage.

Ethereum had been known for having constant scalability issues. Thank God that Vitalik managed to make nodes switch to PoS.

Mempool in Different Blockchain Networks

Variations Across Blockchains

While the basic concept of a mempool is consistent across blockchain networks, its implementation can vary. For example, Bitcoin's mempool might function differently compared to Ethereum's. Each network has its own set of rules and parameters governing how transactions are selected from the mempool.
Here’s a technical deep dive into Bitcoin’s Mempool versus Ethereum, just as you'd expect from us.

Bitcoin's Mempool

  • Implementation and Structure: In Bitcoin's blockchain, the mempool functions as a somewhat straightforward queue. Each node maintains its own version of the mempool, storing transactions that are valid but not yet confirmed.
  • Transaction Selection: Miners typically select transactions based on the fee rate, measured in satoshis per byte. This approach incentivizes users to pay higher fees for faster confirmations, especially during periods of network congestion.
  • Size Limitations: Bitcoin nodes can set limits on their mempool size. When the mempool reaches its maximum capacity, nodes begin dropping transactions with the lowest fee rates. This mechanism prevents mempool bloating and ensures that nodes are not overwhelmed by low-fee transactions during periods of high volume.
  • Child Pays for Parent (CPFP): A unique feature of the Bitcoin mempool is the CPFP mechanism. If a transaction is stuck in the mempool due to low fee, a new transaction can be created that spends the outputs of the unconfirmed transaction and includes a higher fee. This higher fee essentially pushes the first transaction up, incentivizing miners to pick up both. Almost everyone who uses Bitcoin regularly has had to do this at least once.

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Ethereum's Mempool (Transaction Pool)

Implementation and Complexity:

Ethereum's mempool, often referred to as the “transaction pool,” is more complex due to the nature of Ethereum's transactions, which can include not only transfers of value but also interactions with smart contracts.

Gas and Transaction Selection:

Following the rollout of the proof-of-stake algorithm in September 2022, a portion of the gas fee became a reward for staking ETH and participating in validation. Ethereum validators, who verify and process transactions on the network, are rewarded with this fee for staking their ether.

Pending and Queued Transactions:

Ethereum's transaction pool is divided into two main sections: the pending pool, for transactions ready for execution (with a nonce that matches the account state), and the queue, for transactions that are not yet executable (with a higher nonce).

Dynamic Mempool Management:

Ethereum nodes dynamically adjust their mempool size based on the memory allocated to the Ethereum client. This adaptability helps manage memory resources efficiently, especially during network spikes.

Handling Smart Contract Interactions:

Ethereum's transaction pool must account for the complexities of smart contract interactions. Transactions interacting with the same contract may need to be processed in a specific order to maintain state consistency, adding another layer to the challenge of managing the mempool.

Mempool.space: An Insight Tool

Overview and Benefits of Mempool.space

Mempool.space is a legendary tool that provides real-time data about the state of a blockchain's mempool. It offers insights into unconfirmed transactions, fees, and block history. This tool can help you understand when the mempool is congested, allowing you to optimize your transaction times to minimize fees! It also helps miners and validators in decision-making regarding which transactions to prioritize.

Mempool Count in Blockchain Transactions

Understanding Mempool Count

The mempool count refers to the number of transactions currently waiting in the mempool. This number can fluctuate widely depending on network activity and transaction volume. A high mempool count indicates network congestion, while a low mempool count indicates underutilization.


Without a mempool, blockchain technology wouldn't work. That's why it's essential to understand it, not only for blockchain developers but also for users. Its state directly impacts transaction processing times and fees. If there's one thing you should take from this article, it's that you can optimize your fees and transaction times using mempool.space.

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