CONCEPT // MEMPOOL + FEES
What is the mempool?
Where unconfirmed transactions wait, and why fee rate (sat/vB) decides what gets into the next block.
When you send bitcoin, your wallet broadcasts a transaction to the network. Until a miner includes it in a block, it is unconfirmed. Each node keeps its own set of valid, unconfirmed transactions in memory: its mempool.
There is no single global mempool. Nodes hear about transactions at slightly different times and can run different policy settings, so their mempools are similar but rarely identical.
Why fee rate wins
Block space is limited: a block can carry at most 4,000,000 weight units, roughly 1,000,000 virtual bytes (vB). A miner who wants the most fees fills that space with the transactions that pay the most per vB.
When it gets busy
When more transactions are waiting than fit in the next few blocks, low fee-rate transactions wait longer. Replace-by-fee (RBF) lets the sender rebroadcast with a higher fee; child-pays-for-parent (CPFP) lets a recipient spend the unconfirmed output with a high fee so both are mined together.
Facts checked against protocol constants · no invented stats.