What is Mempool in Blockchain?

by Feb 25, 2024Technical Terms0 comments

The mempool, or memory pool, is a critical component of a blockchain network. It acts as a temporary storage space for unconfirmed transactions awaiting inclusion in a block and subsequent addition to the blockchain. This article provides an overview of the mempool’s functions and significance within the blockchain ecosystem, highlighting its role as a queue for pending transactions and its impact on confirmation times and transaction fees.

What is Mempool in Blockchain?

The mempool, short for “memory pool,” is an essential component of a blockchain network. It is a temporary storage area where unconfirmed transactions wait to be included in a block and added to the blockchain.

When a user initiates a transaction, it is broadcasted to the network and enters the mempool of each participating node (computer) in the blockchain network. From there, miners or validators in the network select transactions from the mempool to include in the next block they are trying to mine.

Miners prioritize transactions based on factors such as transaction fees attached to each transaction and the size of the transaction. Transactions with higher fees are often prioritized because miners are financially incentivized to include them in the blocks they mine.

The mempool acts as a queue for pending transactions. Miners periodically select a group of transactions from the mempool and include them in a block. Once a transaction is included in a block and added to the blockchain, it is considered confirmed.

It’s important to note that during times of high network activity, such as when there is a surge in transaction volume or congestion on the blockchain, the mempool can become crowded with a large number of pending transactions. This can lead to longer confirmation times and potentially higher transaction fees if users want their transactions to be prioritized.

How Does Mempool Work?

The journey of a transaction, from initiation to confirmation, encompasses several crucial steps within the blockchain ecosystem. Once a transaction is executed, it is broadcasted to the entire network and enters a mempool, which serves as a temporary storage for pending transactions.

It’s important to understand that mempools are not standardized across blockchain nodes. Each node maintains its own mempool, which can contain a unique set of transactions. Factors such as node configuration and hardware limitations can influence the capacity and timing of transaction inclusion in a node’s mempool.

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How Does Mempool Work?

Furthermore, not all transactions in a mempool are treated equally. Miners and validators possess the discretion to prioritize certain transactions over others when constructing a block. Consequently, users often opt to pay higher gas fees or provide tips as incentives to encourage miners and validators to prioritize their transactions.

This competitive environment for limited blockspace within the mempool has given rise to a dynamic marketplace. Referred to as maximal extractable value (MEV), participants aim to maximize their profits by strategically selecting transactions with the highest potential value to include in a block.

By understanding the intricacies of the mempool and the concept of maximal extractable value, users can navigate the transaction landscape more effectively and optimize their chances of timely transaction confirmation.

Purpose of a Mempool

The purpose of a mempool, or memory pool, is to serve as a temporary storage space for unconfirmed transactions within a blockchain network. It acts as a queue where transactions wait to be included in a block and subsequently added to the blockchain. The main objectives and purposes of a mempool are:

  • Transaction Validation: The mempool allows for the validation of transactions before they are added to the blockchain. Transactions undergo various checks to ensure their integrity, validity, and compliance with the network’s rules and protocols. These checks include verifying the availability of funds, the correctness of transaction signatures, and the adherence to other validation criteria specific to the blockchain.
  • Transaction Prioritization: The mempool facilitates the prioritization of transactions based on factors such as transaction fees, gas prices, or other incentives. Miners or validators can select transactions from the mempool based on these criteria, giving preference to transactions with higher fees or gas prices. This prioritization helps incentivize participants to include transactions in blocks and contribute to the security and operation of the blockchain network.
  • Network Efficiency: The mempool plays a vital role in optimizing the overall efficiency of the blockchain network. By temporarily storing transactions in the mempool, the network can manage and process them in a controlled manner. This helps prevent transaction spam, mitigate network congestion, and maintain a stable and reliable transaction processing environment.
  • Transaction Propagation: The mempool acts as a distribution mechanism for transactions across the network. When a transaction is added to the mempool of one node, it is broadcasted to other nodes in the network. This propagation ensures that all nodes have access to the latest set of pending transactions, allowing for consistent transaction processing and confirmation.
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Why Is My Transaction Still in the Mempool?

If your transaction is still in the mempool and has not been confirmed, there could be several reasons for this:

  • Network Congestion: If the blockchain network is experiencing high traffic or congestion, there may be a backlog of pending transactions in the mempool. Miners have limited block space available and prioritize transactions with higher fees. Your transaction may take longer to be included in a block during periods of high demand.
  • Low Transaction Fee: Miners are motivated to prioritize transactions with higher fees because they earn those fees as rewards for including transactions in blocks. If your transaction has a relatively low fee, it may take longer to be picked up by miners and confirmed.
  • Low Gas Price (Ethereum): In the case of Ethereum or other gas-based blockchains, the gas price you set for your transaction determines its priority. If you set a low gas price, your transaction may be deprioritized by miners who prioritize transactions with higher gas prices.
  • Nonce or Dependency Issues: Some blockchains require transactions to be submitted in a specific order based on the nonce value. If there are dependencies or conflicts with other pending transactions, your transaction may need to wait until those dependencies are resolved.
  • Invalid Transaction: If your transaction does not meet the validation requirements of the blockchain network, such as having insufficient funds or an incorrect signature, it will remain in the mempool without being confirmed.

Mempool Examples

To illustrate the concept of mempools and transaction confirmation times, let’s consider the bitcoin blockchain as an example. 

Here are some important details regarding bitcoin block time: 

On average, there is a 10-minute time interval between two consecutive blocks. This design is intrinsic to the blockchain’s consensus mechanism and ensures the competitiveness of bitcoin mining.

As of 24 August 2022, the average time for confirming a bitcoin transaction was approximately 9.4 minutes, which closely aligned with the average block time. However, one year later, on 24 August 2023, the average confirmation time for bitcoin transactions surged to around 45.6 minutes, as reported by YCharts.

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The primary cause for this increased delay can be attributed to congestion within the bitcoin mempools. The introduction of bitcoin ordinals in January 2023 led to a significant surge in the number of bitcoin transactions.

Providing context, data from The Block indicated that the seven-day moving average (7DMA) of bitcoin transactions nearly doubled from 251,940 transactions a year prior to approximately 492,350 transactions on 23 August 2023. This substantial increase in transaction volume contributed to the congestion observed within the bitcoin mempools.

Important Points to Remember About Mempools

Network congestion represents a significant factor that can cause your transaction to become stuck in the mempool. It is important to note that setting low gas fees can contribute to this issue, as transactions with higher fees are typically prioritized for processing.

Another reason for transaction stagnation in the mempool is insufficient network hashrate. When the hashrate is low, it indicates a lack of computational power within the blockchain network, making it challenging to process transactions in a timely manner.

The time interval between two blocks varies across different blockchains. For example, the average time between two blocks on the Bitcoin network is around 10 minutes, while on Ethereum (ETH), it is approximately 12 seconds.

To expedite a pending transaction, increasing the gas fees associated with it can be effective. By offering higher fees, users increase the likelihood of their transactions being processed promptly. This practice is particularly common during periods of high demand, such as during sought-after NFT mints.

To stay informed about the status of your transaction, you can utilize Block Explorer applications like etherscan for Ethereum. These tools allow you to track and monitor the progress of your transaction within the blockchain network.


In conclusion, understanding the mempool’s functions and its impact on confirmation times and transaction fees is essential for navigating the complexities of blockchain transactions. By considering the challenges of network congestion and optimizing transaction strategies, participants can effectively utilize the mempool and enhance their overall blockchain experience.

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