mempool

The mempool, short for memory pool, is a holding area that exists on every full Bitcoin node. When someone broadcasts a bitcoin transaction to the network, that transaction does not immediately end up in the blockchain. It first arrives at nodes, which check that it is valid, and then add it to their local mempool. Every node maintains its own version of the mempool, so there is no single global mempool, but nodes that are well-connected tend to have a similar set of pending transactions at any given time.

Miners select transactions from the mempool when building a new block. Because each block has a limited size, miners cannot include every pending transaction at once. They prioritise transactions by fee rate, measured in satoshis per virtual byte. A transaction offering a higher fee relative to its size is more attractive to miners because it contributes more revenue per unit of block space. During periods of low demand, almost all transactions get confirmed quickly regardless of fee. During periods of high demand, the mempool fills up and transactions with low fees can wait for hours or even days before being picked up.

The size of the mempool is a useful indicator of network congestion. When many people are transacting simultaneously, the mempool grows and fees rise as users compete for limited block space. When demand decreases, the backlog clears and fees fall. Wallets and fee estimation tools typically monitor mempool conditions to suggest an appropriate fee for the confirmation speed the user wants. Understanding the mempool helps explain why bitcoin transactions are not instant and why fees vary depending on network activity.