What Is a Mempool and How Does It Work—A Simple Explanation

What Is a Mempool

A mempool is a waiting area on a blockchain node where initiated transactions are held until they’re processed. The term comes from the combination of “memory” and “pool.” It refers to the digital queue for transactions that are pending addition to the blockchain.

If you’ve made a crypto transaction, you may have noticed a brief delay before it’s processed. When a transaction is initiated but not yet included in a block, it sits in the mempool. Bitcoin, created by Satoshi Nakamoto, first introduced this concept. Many other blockchain projects, including Ethereum, later adopted the mempool model.

Bitcoin popularized blockchain technology. Satoshi Nakamoto was the first to implement transaction execution with a mempool. Other blockchain projects—including Ethereum—later adopted this model.

Every blockchain has a mempool, though some use different names. For example, Parity refers to its mempool as the “Transaction Queue.” Each blockchain manages its transaction queue differently, but the core principle remains the same.

How Mempool Size Is Determined

Each transaction in the mempool is a data packet, usually just a few kilobytes. The total size of all these transactions determines the mempool’s size.

A larger mempool signals a higher number of transactions awaiting confirmation. Bitcoin’s mempool size often rises with increased price volatility, as more trading and transfers take place during active market movements.

Interesting! Nodes can set limits on their mempool size. If the mempool exceeds this limit, nodes may require a minimum fee for processing new transactions. This protects the network from spam and helps maintain stability.

The Role of the Mempool in Blockchain Transactions

The mempool is central to how blockchain nodes operate. For a transaction to be recorded on the blockchain, it must first enter a block. Not every node can create blocks. In Proof-of-Work systems like Bitcoin, miners are responsible for adding transactions to blocks. In Proof-of-Stake systems like Ethereum, validators perform this function.

Once a transaction is created, users wait for a validator or miner to approve and add it to a block, which is then recorded on the blockchain. This doesn’t happen instantly. While the transaction awaits verification, addition to a block, and recording on the blockchain, it remains in the mempool. The wait time depends on network congestion and transaction priority.

The mempool acts as a buffer, ensuring transactions are processed in an orderly and secure way. Without it, blockchain networks would struggle to handle high volumes and maintain data integrity.

How the Mempool Works

One blockchain can have multiple mempools—each node maintains its own space for pending transactions. Together, all these individual mempools form one collective mempool across the network.

When a user initiates a transaction, it’s sent to a node. The node adds the transaction to its mempool and queues it for validation (to verify data accuracy). After passing validation, the transaction’s status changes to pending. At this stage, miners or validators can add these transactions to new blocks, which are then appended to the blockchain.

Validation checks include confirming the sender’s balance, verifying digital signatures, and ensuring compliance with network rules. Only transactions that pass validation move forward.

When a transaction is recorded on the blockchain, nodes receive this update. They can then remove the corresponding transaction from their mempool. This synchronization ensures all nodes have consistent information about transaction status.

The Lifecycle of a Cryptocurrency Transaction and the Role of the Mempool

To illustrate how the mempool works, let’s say you want to send 0.01 BTC to a friend.

1. First, you need your friend’s wallet address. Set this as the recipient when initiating your transaction, approve the system’s transfer fee, and click “send.”

2. Your transaction is added to the nearest mempool and marked as queued for processing. At this point, it’s registered but not yet verified.

3. The transaction is broadcast to other network nodes, allowing them to check compliance with network rules. This broadcasting ensures the entire network is aware of your transaction.

4. If the transaction passes all checks, it moves from the general queue to the waiting pool (for block inclusion and blockchain recording). If there’s an issue, you’ll receive a notification that the transaction is unconfirmed.

5. A miner or validator selects the transaction—usually based on fee priority—and adds it to a new block, recording it on the blockchain.

6. The miner or validator then broadcasts the new block to the network. Other nodes receive information about the included transactions.

7. Nodes can now remove your transaction from their mempools, freeing up space for new entries.

8. All done! The transfer is complete, and your friend receives 0.01 BTC. The transaction is now permanently recorded on the blockchain and cannot be changed.

Mempool Overload

Mempool overload happens when the number of requested transactions exceeds what can fit in a single block. Several factors can cause the mempool to slow down:

• Network congestion. A surge in transactions can exhaust block space, overloading the mempool. For example, a typical Bitcoin block holds about 3,150 transactions. If pending transactions far exceed this for hours, the mempool and network become overloaded—leading to longer wait times and higher fees.

• Events or news. Token launches, airdrops, or celebrity crypto endorsements can trigger sudden transaction spikes, overloading the mempool. Major announcements or high price volatility also lead to surges in activity.

• Forks or network upgrades. When the network undergoes a fork or upgrade, nodes may be occupied with updates and unable to process transactions at normal speed, causing mempool overload.

By understanding these causes, developers can adjust their projects to minimize impact. Solutions include increasing block size, implementing layer-two protocols, or optimizing consensus algorithms.

Transaction Priority

Transaction fees are a key factor in determining processing order within the mempool. Miners and validators select transactions for new blocks based on potential profit. The higher the fee a user pays, the faster their transaction is processed.

So, how long does it take to confirm a Bitcoin transaction? It depends on mempool congestion and the fee paid. Lower congestion and higher fees mean faster processing. High congestion or low fees can delay confirmation for Bitcoin or other cryptocurrencies.

This priority system creates a market dynamic: users who need fast confirmation can pay more, while those willing to wait can pay less. This approach helps the blockchain network allocate limited resources efficiently.

FAQ

What Is a Mempool and How Does It Work—Simply Explained

A mempool is a temporary area for unconfirmed blockchain transactions. Each node maintains its own mempool, storing transactions awaiting verification before block inclusion. The mempool manages transaction fees and prioritizes higher-fee transactions for faster confirmation.

How Does the Mempool Work? How Do Transactions Enter the Mempool?

When a user sends a transaction, a node receives and places it in its mempool—queued for verification. Transactions stay in the mempool until a miner adds them to the next block.

Why Do Transactions Sometimes Wait in the Mempool for a Long Time?

High network congestion and limited block space can keep transactions in the mempool for long periods. Lower-fee transactions are deprioritized, so they take longer to confirm.

What Is the Impact of Mempool Congestion on Transaction Fees?

Mempool congestion increases transaction fees. Miners prioritize transactions with higher fees, so users must pay more to ensure quick processing when the network is busy.

How Can I Check the Current Mempool Status and Pending Transactions?

You can use blockchain explorers like Mempool.space to see real-time mempool status. These tools show mempool size, fee levels, and the number of transactions awaiting confirmation—helping you optimize timing and costs.

What Happens If the Mempool Is Full? Will Transactions Be Deleted?

When the mempool is full, nodes remove the lowest-fee transactions to make space for higher-fee ones. Removed transactions can be resent when congestion eases or if the fee is increased.

How Do Transaction Priority and Gas Fees Relate to the Mempool?

Gas fees determine transaction priority within the mempool. Miners process transactions with higher gas fees first. The more you pay in gas fees, the higher your transaction’s place in the mempool queue.