> For the complete documentation index, see [llms.txt](https://docs.leather.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.leather.io/learn/mempool.md).

# What is a Mempool?

The mempool is where Bitcoin transactions wait before a miner puts them in a block, and why some wait longer.

The mempool (memory pool) is the set of valid, unconfirmed transactions a Bitcoin node holds while they wait to be mined. Each node keeps its own mempool, so their contents differ slightly.

## From wallet to block

1. **Create:** your wallet builds and signs the transaction.
2. **Broadcast:** the wallet sends it to a node, which checks it and relays it to other nodes. It now waits in their mempools.
3. **Confirm:** a miner picks transactions from its mempool, puts them in a block and finds a valid proof of work. When the block joins the chain, the transaction has one confirmation.

## Why a transaction waits

* **Low fee rate:** miners fill blocks with the transactions that pay the most per unit of size. A low fee rate waits until demand drops.
* **Congestion:** a block holds at most 4 million weight units. When more transactions arrive than fit, the mempool grows and the fee rate needed to get in rises.
* **Slow blocks:** blocks arrive every 10 minutes on average, but the gap varies. A long gap leaves transactions waiting.

A node can drop a transaction that waits too long or pays too little when its mempool is full.

To speed up a waiting transaction, see [What is Replace-By-Fee (RBF)?](/learn/replace-by-fee-rbf.md) and [Understand Pending Transactions](/transactions/pending-transactions.md).