What Is a Mempool? How Pending Transactions Work

Have you sent cryptocurrency and waited a long time for it to arrive? The transaction may be waiting in the mempool. This guide explains what that waiting room is and why transactions can stall.

What is a mempool?

Mempool is short for memory pool: a waiting area for transactions that have not yet been included in a block and remain unconfirmed. On networks such as Bitcoin, broadcasting a transaction does not immediately confirm it. It first enters mempools and waits for a miner, or a validator on other networks, to select it.

Each node, a computer participating in the network, maintains its own mempool, so its pending-transaction list can differ slightly from other nodes'. The mempool is therefore closer to many independently maintained waiting lists than one enormous shared warehouse.

Transaction order and fee priority

When transactions pile up, which ones go first? The key factor is the transaction fee. A block has limited capacity, so miners generally prioritize transactions offering higher fees. Paying more acts like paying to move ahead in the queue.

Why transactions get stuck and what can help

A transaction that remains unconfirmed for hours often has a fee too low to compete with other pending transactions. This is common when a low-fee transaction is sent during congestion.

• RBF, or Replace-By-Fee: Some wallets allow a transaction to be replaced and rebroadcast with a higher fee.
• CPFP, or Child-Pays-For-Parent: Spending received coins in a new, high-fee transaction can encourage confirmation of its parent transaction as well.
• Waiting: A transaction may confirm when congestion clears. The original guide also describes transactions eventually dropping out of a mempool and the funds becoming available again in the wallet.

Wallet support varies. Before sending, check the fee-adjustment options in your wallet.

The connection between mempools and MEV

Publicly visible pending transactions create opportunities for others to inspect them and seek profit. This connects to MEV, or Maximal Extractable Value. For example, when someone broadcasts a large purchase, another participant may see it and arrange for their own transaction to execute first, capturing a price difference. Such activity is especially common on smart-contract networks such as Ethereum.

Beginners do not need to master MEV immediately, but should remember that transactions in a public mempool can be seen before confirmation. On-chain analysis tools can show live mempool conditions, including pending transaction counts and suggested fees.

Key points

A mempool is a waiting room for unconfirmed transactions, where fees largely determine priority. A stalled transaction may have offered too little. RBF or CPFP may help, or you can wait for congestion to ease.

This article is for information and is not an investment recommendation. Cryptocurrency transactions and network fees vary substantially with congestion and involve risk of loss. Setting a low fee does not guarantee eventual confirmation. Be cautious of information promising unconditional speed or guaranteed profits.