A mempool is a temporary storage area for unconfirmed transactions in a blockchain network. Each node keeps its own copy of this queue. A transaction remains there until it is included in a block or its waiting time expires. Understanding this mechanism helps manage fees and avoid stuck payments.

What Is a Mempool in Simple Terms

What a mempool is is a relevant question for everyone who works with cryptocurrency. Imagine a queue at a checkout: each transaction takes its place and waits for its turn. A miner or validator selects profitable transactions from the queue and packs them into a block. The rest wait for the next round.

The size of the queue changes constantly. During periods of high network activity, the mempool grows to hundreds of thousands of transactions. During quiet periods, it is almost empty.

How the Mempool Works in Blockchain

The blockchain mempool works according to a simple logic: a transaction passes basic validation and enters the node’s local queue. The node then broadcasts it to its neighbours — and within seconds the transaction spreads across the network.

Each node stores its own version of the mempool. There is no single global mempool in Bitcoin or Ethereum — this is an important architectural decision. Different nodes see a different set of transactions at the same moment.

Priority is determined by the fee per byte: sat/vByte in Bitcoin and gas price in Ethereum. Transactions with a high fee get into a block faster.

Why the Mempool Can Become Congested

Congestion occurs when there are more transactions on the network than it can process in time. In Bitcoin, for example, a block holds roughly 1–4 MB of data and is created once every 10 minutes. If activity increases, the queue starts growing and gradually becomes overloaded.

Most often this happens because of specific events. This was the case in May 2023 after the launch of the Ordinals protocol — the number of transactions increased sharply, the queue exceeded 400,000, and fees reached 30–50 sat/vByte. A similar situation was observed in Ethereum during NFT hype cycles and the launch of DeFi protocols.

How the Mempool Affects Fees

The relationship is direct: the larger the queue, the higher the competition for space in a block. A sender who is interested in fast confirmation raises the fee. Miners choose transactions with the maximum return per byte.

Fee estimation in wallets is not exact — it is based on analysis of the current network load and recent blocks. Therefore, during periods of sudden activity spikes, the actual confirmation time may differ from the expected one.

As a rule, wallets offer three priority levels:

• High - confirmation in the next block (approximately 10 min. Bitcoin, ~1–3 min. Ethereum)
• Medium - confirmation within 30–60 minutes
• Low - fee savings if the user is ready to wait several hours

Under unstable load, even high priority does not always produce a quick result. In such cases, users more often rely on the current mempool situation rather than only on wallet recommendations.

For exchange services, it is important to take the state of the queue into account and adjust fees depending on the load. This makes it possible not to overpay during quiet periods and to reduce the likelihood of delays when activity increases.

Mempool in the Bitcoin Network

The Bitcoin mempool is the most studied and publicly tracked transaction queue. The average size during a quiet period is 5,000–15,000 transactions; during peak periods, it reaches hundreds of thousands.

According to the Bitcoin Core documentation, the mempool size on nodes is limited to 300 MB by default. If the limit is exceeded, the node evicts transactions with the lowest fee.

The Bitcoin mempool is also sensitive to halvings: after the mining reward is reduced, miners become more selective when choosing transactions based on profitability.

Mempool in the Ethereum Network

In Ethereum, a similar structure is called txpool or pending pool. After the transition to Proof-of-Stake and the implementation of EIP-1559, the mechanism changed: the base fee is burned, while tips go to the validator.

According to data from Etherscan: the number of pending transactions can reach tens of thousands under high network load. After the implementation of EIP-1559, the fee mechanism changed, introducing a base fee and a priority fee, which improved the predictability of transaction costs.

How to View the Mempool in Real Time

The Bitcoin mempool online can be conveniently tracked through specialised tools. The most useful are:

Mempool.space is the most detailed tool: it shows the queue broken down by fee ranges and predicts confirmation time.

What to Do If a Transaction Gets Stuck in the Mempool

A transaction gets stuck when its fee is below the current network minimum. There are two ways to speed up confirmation:

RBF (Replace-By-Fee) - replacing the transaction with a new one that has a higher fee. It works if the original transaction has the RBF flag. It is supported by most modern Bitcoin wallets.

CPFP (Child-Pays-For-Parent) - creating a child transaction with a high fee. The miner includes both transactions because the child depends on the parent. This method is useful for the recipient who does not have access to the original wallet.

In Ethereum, the equivalent of RBF is resending a transaction with the same nonce but a higher gas price.

Advantages and Disadvantages of the Mempool

The mempool performs a key buffering function: without it, the network would lose transactions under overload. The queue gives time for the propagation and verification of data.

Advantages:

1. Buffering under peak network load - the mempool smooths activity spikes and prevents transaction loss when blockchain throughput is limited. Without it, users would face mass transaction failures.
2. Flexible fee management - the sender can adapt their strategy: speed up a transaction by increasing the fee or save money by choosing low priority. This creates a competitive environment among network participants.
3. Transparency and analytics - open access to mempool data makes it possible to forecast network load, estimate transaction costs, and optimise sending time. This is especially important for exchange services and payment platforms.
4. Error correction mechanisms - the availability of RBF and CPFP makes it possible to “revive” stuck transactions without a complete loss of funds or the need to wait for the mempool to clear.

Disadvantages:

• Unpredictability during peak periods - when the load rises sharply, fees can increase multiple times within a short period, which complicates transaction cost planning.
• Confirmation delays - transactions with a low fee may remain in the mempool for hours or even days, especially under network congestion.
• Risk of MEV and front-running - the public nature of the mempool allows bots to analyse incoming transactions and insert their own with higher priority, extracting profit at the expense of other participants.
• Data fragmentation - because there is no single global mempool, different nodes see different pictures of the queue, which leads to differences in confirmation time estimates.
• Load on nodes - when the mempool becomes congested, memory and resource consumption increases, which can lead to the eviction of low-fee transactions and a poorer user experience.

Conclusion

The mempool is a basic transaction routing mechanism in blockchain networks. Its state directly determines confirmation speed and fee levels. Before sending a large payment, it is worth checking network congestion — Mempool.space provides an accurate real-time picture. For operators of exchange platforms, including services based on BoxExchanger, monitoring the transaction queue becomes part of the operational routine when setting up automatic payouts.