The Bitcoin mempool (memory pool) is the waiting room for unconfirmed transactions. When you broadcast a transaction, it enters the mempool and waits until a miner includes it in a block. Miners select transactions that maximize their revenue, prioritizing those with the highest sat/vB rate. During calm periods, the mempool is nearly empty and fees stay near 1 sat/vB. During congestion, thousands of transactions pile up and rates can exceed 500 sat/vB.
If your transaction is stuck in the mempool with a fee that is too low, two tools can help. Replace-By-Fee (RBF) allows you to rebroadcast the same transaction with a higher fee, replacing the original in the mempool. Child-Pays-For-Parent (CPFP) works by creating a new outgoing transaction from an unconfirmed output, attaching a high fee that incentivizes miners to confirm both transactions together. Most modern wallets support both methods.
Fee spikes occur whenever demand for block space exceeds supply. Major causes include: Bitcoin price surges attracting new users; Ordinals and inscription activity occupying large amounts of block weight; Exchange batch withdrawals delayed by low fees causing re-broadcast storms; Halving-related hype events. When analyzing the mempool, look at the size in MB waiting above a given fee threshold. If 5 MB is waiting at 10+ sat/vB, you may need to wait several blocks even at that rate.
