2.4 The Mempool & Fee Markets

When you send a Bitcoin transaction, it doesn't go straight into a block. First it sits in a kind of global waiting room called the mempool — every node on the network keeps its own copy of pending transactions.

Roughly every 10 minutes a miner publishes a new block. Block space is limited, so miners naturally pick the transactions offering the most fee for their size. The unit is 'sats per virtual byte' (sats/vB) — basically a price per ounce of space.

When lots of people are sending Bitcoin, the auction heats up and fees rise. When it's quiet, you can get in for very little. Wallets show suggested fee levels (fast / medium / slow), and you can almost always pick a cheaper option if you're not in a hurry.

If your transaction is stuck, two tools can save you: Replace-By-Fee (RBF) re-broadcasts the same transaction with a higher fee, and Child-Pays-For-Parent (CPFP) spends the unconfirmed change output with a high-fee follow-up that drags the original along.

Fees aren't just a nuisance — they're Bitcoin's long-term security paycheck. As the new-coin reward to miners shrinks every four years, fees will become how the network keeps paying for its own protection.