Mempool

When you send bitcoin, your wallet passes the transaction to the network, and each node that accepts it holds it in memory, in what Bitcoin Core calls its memory pool, or mempool, until a miner puts it in a block. There’s no single mempool: each node keeps its own.

Blocks have limited room and come about every 10 minutes, so miners take first the transactions that pay more per byte. When more are waiting than the next blocks can hold, the ones paying less wait longer, and nothing guarantees they’ll ever be confirmed. By default, Bitcoin Core keeps its mempool under 300 megabytes, pushes out whatever pays too little to stay when it’s full, and drops any transaction still waiting after two weeks. A transaction that’s dropped never happened: the coins stay in the sender’s wallet, because they never left it.

That’s why a network fee that was enough yesterday can be too low today. On May 7, 2023, more than 400,000 transactions were waiting, more than ever before, according to The Block. Binance, a large exchange, paused bitcoin withdrawals twice that day, for about two hours each time: its customers’ withdrawals were stuck, because the fees it had set didn’t anticipate the surge. It raised the fees and resumed.

Sites such as mempool.space show what’s waiting and what it pays, and many wallets can raise the fee of a transaction that’s stuck. The guide on what buying bitcoin really costs covers the network fee among the other costs.