What is a mempool and how does it work in simple terms

What Is a Mempool?

A mempool is a "waiting room" on a blockchain node, where transactions that have been initiated but not yet processed are temporarily stored. The term combines "memory" and "pool," referencing a digital area meant for queuing transactions before they're added to the blockchain.

When users initiate crypto transactions, they may experience delays while the system processes the operation. Once a transaction is started but not yet finalized, it resides in the mempool. This interim state is essential to how blockchain networks handle transactions.

Bitcoin was the first project to popularize blockchain technology. Its creator, Satoshi Nakamoto, introduced the concept of processing transactions through a mempool. Other blockchain projects, including Ethereum, later adopted this model.

All blockchains have mempools, though some use different names. For example, the Parity blockchain project refers to its mempool as the "Transaction Queue." Regardless of the label, the purpose remains the same—temporarily storing unprocessed transactions.

How to Determine Mempool Size

Each transaction added to the mempool is a data packet typically no larger than a few kilobytes (KB). The total size of these transactions determines the mempool's size, serving as a key indicator of the blockchain network’s status and overall load.

A large mempool means many transactions are waiting for confirmation. Tracking mempool size trends helps forecast transaction processing speed and estimate the optimal transaction fee for priority execution.

Monitoring charts show that Bitcoin’s mempool size often spikes when crypto prices swing sharply—either up or down—reflecting increased trading activity during periods of volatility.

Interesting! Nodes can set mempool size limits. If the threshold is exceeded, nodes may enforce a minimum fee for transactions. This helps protect the network from overload and spam attacks.

The Role of the Mempool in Blockchain Transactions

Mempools play a critical role in how blockchain nodes operate. For a transaction to be recorded on the blockchain, it must first be included in a block. Not all nodes can create blocks; this depends on the network’s consensus mechanism.

For example, in Proof-of-Work blockchains such as Bitcoin, only miners can add transactions to blocks. Miners use computational power to solve cryptographic puzzles and build new blocks. In Proof-of-Stake systems such as Ethereum, validators who stake their tokens serve this role.

After a transaction is created, the user must wait for a validator or miner to approve it for block inclusion and subsequent recording on the blockchain. This process isn't instant—a transaction must be verified and consensus must be reached across the network. During this entire period—while awaiting verification, block inclusion, and blockchain recording—the transaction stays in the mempool.

How the Mempool Works

One blockchain can have many mempools, as each node has its own digital space for holding transactions pending validation and inclusion in the blockchain. Together, all these mempools across individual nodes form a collective, decentralized mempool, ensuring the network's reliability and resilience.

When a user submits a transaction, it’s sent to a node. The node adds the transaction to its mempool and queues it for validation, checking digital signatures, confirming the sender’s balance, and ensuring protocol compliance.

Once validated, the transaction enters a pending state. Only these validated transactions can be picked by miners or validators for block inclusion. Transaction selection depends on several factors, with the fee size playing a key role.

When a transaction is added to the blockchain, nodes are notified. At that point, nodes can remove the transaction from their local "waiting room," keeping mempools synchronized across the network.

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

To illustrate the mempool’s function and a transaction’s lifecycle, suppose you want to send 0.01 BTC to a friend. Here’s how the process unfolds step by step:

1. Transaction initiation. First, you obtain your friend’s crypto wallet address and enter it as the recipient. You may need to confirm the suggested transaction fee. Finally, you click "send."

2. Adding to the mempool. The transaction is sent to a mempool and marked as pending. At this point, it’s not yet part of the blockchain.

3. Broadcast and verification. The network broadcasts your transaction to other nodes, allowing them to verify protocol compliance.

4. Validation. If nodes approve the transaction, it moves from the general queue to a waiting group for block inclusion. If there’s an issue, you’ll be notified that the transaction wasn’t confirmed.

5. Inclusion in a block. A miner or validator adds your transaction to a new block and records it on the blockchain. Transaction selection often depends on the fee size.

6. Information propagation. The miner or validator broadcasts the block to other nodes, so all nodes learn which transactions were included.

7. Mempool cleanup. At this stage, nodes remove your transaction from their mempools.

8. Completion. That’s it—the transfer is complete. Your friend receives 0.01 BTC, and the transaction is permanently recorded on the blockchain.

Mempool Congestion

Mempool congestion occurs when the number of pending operations exceeds the number of transactions that can fit in a single block. This can significantly slow processing and increase transaction fees. Key factors that cause mempool congestion include:

• Network congestion. A surge in transaction volume can fill available block space and overload the mempool. For example, a typical Bitcoin block holds about 3,150 transactions. If pending transactions far exceed this for several hours, both the network and mempools become congested. Wait times for confirmations can stretch from minutes to hours or even days.

• Events or news. Industry events—such as token launches, airdrops, or celebrity crypto endorsements—can trigger sudden spikes in transaction demand, leading to mempool congestion. This has happened during high-profile ICOs and new NFT project launches.

• Forks or network upgrades. Network changes like forks or protocol upgrades can instantly congest the mempool as nodes focus on updating and synchronizing with the latest version. Network performance typically dips during such periods.

Transaction Priority

Transaction fees are a primary factor in determining the processing order within the mempool. Miners and validators select transactions for block inclusion based on potential profit. In short, the higher the fee a user pays, the faster the transaction is processed.

This creates a fee market, where users compete for space in the next block. During high network activity, fees can rise dramatically compared to normal conditions. Some wallets offer dynamic fee recommendations based on the current mempool status.

As a result, Bitcoin transaction confirmation times depend on mempool load and the fee you set. Lower network load and higher fees mean faster confirmation. Conversely, heavy congestion and low fees can slow confirmations for Bitcoin or other cryptocurrencies. In some cases, very low-fee transactions may not be processed at all and are removed from the mempool after a set period.

FAQ

What is a mempool and how does it work in simple terms?

A mempool is a temporary storage space for unconfirmed blockchain transactions. Each transaction includes a fee, which determines its processing priority—higher fees mean faster confirmation.

Why do transactions sometimes wait a long time in the mempool?

Transactions remain in the mempool due to low fees or network congestion. Validators prioritize transactions with higher fees, and heavy transaction volume can cause delays.

How does the transaction fee affect a transaction's position in the mempool?

The transaction fee sets processing priority in the mempool. Higher fees move the transaction up the queue and accelerate processing. Transactions with higher fees are confirmed first.

Can a transaction be removed from the mempool?

No, you can't manually remove a transaction from the mempool. The mempool temporarily holds unprocessed transactions until they're included in the blockchain. However, you can replace it with a higher-fee transaction or wait for the retention period to expire.

How does mempool size affect blockchain network speed?

A larger mempool increases network load, but an optimally sized mempool allows for faster transaction processing without congestion, improving overall blockchain efficiency.

What's the difference between mempools in different cryptocurrencies?

Mempools differ across cryptocurrencies in size, processing speed, and fee structure. Bitcoin’s mempool is often congested, while Ethereum typically processes transactions faster. Each blockchain sets its own rules and limits for mempool management.