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Blocks & network · 2 min read

What is the mempool?

By Oliver James Whitmore · Updated 20 October 2026

In one sentence

The mempool, short for memory pool, is the waiting area in which a node keeps valid transactions that have been broadcast but not yet included in a block.

Waiting transactions in a mempool; the block producer picks those paying more per unit of space first.Mempool (one node)fee 3fee 9fee 1fee 6fee 2fee 8fee 4fee 7fee 1Next block (limited space)fee 9fee 8fee 7Low-fee transactions wait longer when the network is busy
Waiting transactions in a mempool; the block producer picks those paying more per unit of space first.

How it works

When a node receives a new transaction and finds it valid, it stores it in memory and passes it on. That collection of waiting transactions is the node's mempool. There is no single, official mempool: every node has its own. They are usually very similar, because gossip spreads transactions quickly, but they are not always identical — two nodes may have heard different things at a given moment.

Block producers build each new block from their own mempool. Space in a block is limited, so they generally prefer transactions that pay more per unit of space: per virtual byte in Bitcoin, or per unit of gas in Ethereum. When many people send at once, the pool grows and low-fee transactions wait longer.

Mempools cannot grow forever. Nodes set a size limit — 300 MB by default in Bitcoin Core — and drop the lowest-paying transactions when it is reached. Transactions that wait too long can also expire; Bitcoin Core's default is two weeks. Many wallets let a sender replace a waiting transaction with a version that pays a higher fee, a mechanism described for Bitcoin in BIP-125.

A simple example

On a quiet day, a transaction with a modest fee is picked up in the next block. During a busy period, the same fee places it behind thousands of better-paying transactions; it may wait for hours, or be dropped and need to be sent again. Nothing about the transaction itself changed — only the queue around it.

Common misconception

Misconception: There is one mempool, and websites show exactly what is in it.

In fact: Each node has its own mempool. A website that shows "the mempool" is showing the view of the node or nodes it runs, which is close to, but not the same as, everyone else's.

Where you meet it

  • "Unconfirmed" or "pending" labels in wallets and block explorers.
  • Fee estimates, which are calculated from what is waiting.
  • Discussions of busy periods on a network.
  • Replace-by-fee and similar tools that let a sender raise the fee on a transaction that is still waiting.

Sources

This card explains a technical concept for general education. It is not a recommendation to acquire, sell or hold any crypto-asset, and it is not financial advice.