What is a mempool and how does it work in plain language

Mempool Fundamentals

If you’ve ever made a crypto transaction, you may have noticed a delay before it’s finalized. Once you initiate a transaction but it hasn’t been processed yet, it sits in what’s called the mempool.

The mempool is a digital “waiting room” hosted on a blockchain node. This digital space holds transactions that have been initiated but not yet processed. The term “mempool” is a blend of “memory” and “pool,” highlighting its purpose as a holding area for transactions queued for inclusion in the blockchain.

Every blockchain has its own mempools, though some use different names. For example, the Parity blockchain project calls its mempool the “Transaction Queue.” The terminology may differ, but the underlying mechanism is the same—reflecting the diverse vocabulary in the crypto ecosystem.

How Mempool Size Is Determined

Each transaction added to the mempool is a chunk of data, typically just a few kilobytes (KB) in size. The combined size of all these transactions defines the total mempool size.

A large mempool means there are many transactions waiting for confirmation. This serves as a key indicator of network congestion, helping users gauge how quickly their transactions might be processed.

On activity charts, you’ll often see Bitcoin’s mempool size spike during periods of high price volatility—both rallies and drops. That’s because users move assets more actively during major price swings, aiming to lock in profits or minimize losses.

Nodes can set limits on their mempool size. If the threshold is exceeded, the node may require a minimum transaction fee. This acts as a safeguard to prevent network overload and keep the system running smoothly.

The Mempool’s Role in Blockchain Transactions

Mempools are essential to how blockchain nodes operate. To record a transaction on the blockchain, it first needs to be added to a block. Not every node can create blocks.

For example, in Proof-of-Work blockchains like Bitcoin, only miners have the authority to add transactions to blocks. Miners expend computing power solving complex problems, and if successful, they earn the right to create a new block.

In Proof-of-Stake blockchains—like Ethereum—validators are responsible for adding transactions to blocks. They lock up a set amount of cryptocurrency as collateral and gain the right to confirm transactions in proportion to their stake.

After you create a transaction, you must wait for a validator or miner to approve it for block inclusion and eventual recording on the blockchain. This isn’t instantaneous because the network needs to verify and reach consensus. During this entire process—while the transaction awaits verification, inclusion in a block, and final recording—it resides in the mempool.

How the Mempool Operates

Importantly, a single blockchain can have many mempools. Each node maintains its own digital space for storing transactions pending validation and block inclusion. Collectively, all these mempools across private nodes form one large, distributed mempool.

This decentralized structure makes the network resilient: even if some nodes go offline, transaction data persists on other nodes throughout the system.

When you create a transaction, it’s sent to a node. The node adds it to its mempool and places it in the validation queue—checking data integrity, verifying digital signatures, confirming the sender’s balance, and ensuring the transaction fits protocol requirements.

Once validated, the transaction enters a pending state. These pending transactions are what miners and validators can add to new blocks for inclusion in the blockchain.

After a transaction is added to the blockchain, nodes are notified. At that point, they can clear the now-redundant transaction data from their “waiting rooms.” This is how mempool synchronization happens across the network.

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

To illustrate the mempool’s function and the transaction lifecycle, imagine you want to send 0.01 BTC to a friend.

1. First, you need your friend’s crypto wallet address. Enter this as the recipient when initiating the transaction. You may also need to accept the transfer fee specified by the system. Finally, click “send.”

2. Your transaction enters the nearest mempool and is flagged as pending. At this point, it’s not yet recorded on the blockchain but is registered in the system.

3. The transaction is then broadcast to other nodes, which check and validate it for compliance with network rules. This decentralized validation ensures no single authority controls the process.

4. If the nodes validate your transaction, it moves from the general queue to the waiting group for block inclusion and eventual recording on the blockchain. If something’s wrong, you’ll get a notification that the transaction was not confirmed.

5. A miner or validator selects your transaction, adds it to a new block, and records it on the blockchain. The decision often depends on the transaction fee.

6. The miner or validator who added your transaction then broadcasts the new block to other nodes. As a result, network nodes become aware of the transactions included in the block.

7. At this point, nodes can remove your transaction data from the mempool, freeing up space for new transactions.

8. That’s it! The transfer is complete—your friend receives 0.01 BTC, and the transaction is permanently recorded in the blockchain ledger.

Mempool Congestion

Mempool congestion occurs when the volume of pending transactions exceeds what can fit in a single block. This can significantly slow transaction processing and drive up fees.

Several factors can cause the mempool to slow down:

• Network congestion. A surge in transactions can fill blocks to capacity, causing the mempool to back up. For instance, the average Bitcoin block holds about 3,150 transactions. If unconfirmed transactions greatly exceed this number for several hours, both the network and mempools become congested. Users must then wait longer or pay higher fees.

• Events or news. Major events or news—such as token launches, airdrops, or celebrity crypto endorsements—can trigger sudden spikes in transaction demand. These activity surges often lead to mempool congestion and have historically occurred during popular NFT launches or sharp price swings.

• Forks or network upgrades. Network changes like forks or protocol upgrades can instantly clog the mempool. Nodes may be busy updating software, reducing their ability to process transactions temporarily. Users may also rush to transact before or after upgrades, adding to the load.

Transaction Priority

Transaction fees are a key factor in determining processing order within the mempool. Miners and validators select which transactions to include in the next block based on potential profit. In short, the higher the fee you’re willing to pay, the faster your transaction is processed.

This dynamic creates a fee market where users compete for block space. During times of network congestion, fees can spike tenfold compared to normal periods.

How long you wait for a Bitcoin transaction confirmation depends on both mempool congestion and your fee. Lower congestion and higher fees lead to faster processing. High congestion and low fees, on the other hand, can slow confirmations for Bitcoin or any other cryptocurrency.

Some wallets offer dynamic fee calculations, analyzing the current mempool and recommending an optimal fee for quick confirmation. Users can also monitor mempool status through blockchain monitoring services to pick the best time for transactions.

FAQ

What is a mempool and why does a blockchain need it?

The mempool is a temporary holding area for unconfirmed transactions. It collects transactions before they’re added to a block, ensuring orderly fund movement and network optimization. The mempool is vital for blockchain stability.

How do transactions enter the mempool and how are they processed?

Transactions spread across the network via nodes and enter the mempool to await processing. Miners verify them based on fees and add them to the blockchain in order of priority.

Why does mempool size affect transaction fees?

A large mempool means more competition among transactions. Miners prioritize those with higher fees, since fees are their reward. More pending transactions lead to higher fees for priority processing.

How long can a transaction stay in the mempool?

A transaction remains in the mempool until it’s included in a block. If not included, it may stay for 1–2 hours before being removed. Funds aren’t automatically returned for unconfirmed transactions.

What’s the difference between the mempool and the blockchain?

The mempool is where transactions wait before being written to the blockchain. The blockchain is the permanent record of confirmed transactions. The mempool is temporary; the blockchain is a lasting data store.

How can you check your transaction’s status in the mempool?

Use your transaction hash in a blockchain explorer to check status, confirmation count, and queue position. If there are no confirmations, the transaction is still pending network processing.