Mempool
The mempool (memory pool) is the waiting area where unconfirmed Bitcoin transactions sit before being included in a block by miners. Each full node maintains its own mempool, and transactions remain there until a miner selects them or they expire.
How It Works
When you broadcast a Bitcoin transaction, it propagates across the network and lands in the mempool of each full node that receives it. The mempool is not a single shared pool — every node maintains its own version based on the transactions it has seen and its own policy rules. This means mempools can differ slightly between nodes.
Miners draw from their mempool when constructing new blocks, typically selecting transactions with the highest fee rates (sats per vbyte) first. During periods of high demand, the mempool grows and fee rates spike as users compete for limited block space. During quiet periods, even low-fee transactions get confirmed relatively quickly. Monitoring mempool conditions before sending a transaction helps you choose an appropriate fee rate.
Transactions can remain in the mempool for extended periods if their fee rate is too low. Most nodes drop transactions from their mempool after a default timeout (typically 14 days in Bitcoin Core). If a transaction is dropped, the UTXOs it attempted to spend become available again. Features like replace-by-fee (RBF) allow you to bump the fee on a stuck transaction, giving you control over confirmation priority.
Key Points
- Each full node maintains its own independent mempool
- Miners prioritize transactions with higher fee rates (sats/vbyte)
- Mempool size fluctuates with network demand — monitor before transacting
- Unconfirmed transactions are not settled and can be replaced or dropped
- Replace-by-fee (RBF) lets you increase fees on stuck transactions