Mempool

The mempool (short for memory pool) is the set of valid, broadcast transactions that a blockchain node is holding while they wait to be included in a block.

Every node keeps its own mempool, so there is no single official one, but nodes relay transactions to each other within seconds and their pools usually look almost the same. Miners pick transactions from their mempool to build the next block, and on Bitcoin they fill it with the highest fee rates first, measured in satoshis per virtual byte. When more transactions are waiting than the next block can hold, the lowest bids sit in the mempool until demand drops or they get replaced.

The mempool has limits. Bitcoin Core nodes cap it at 300 MB by default and evict the lowest-fee transactions when it fills, and they drop anything still unconfirmed after two weeks. A transaction that falls out of most mempools never happened on-chain: the coins stay in the sender's wallet and can be spent again. That is why a stuck transaction is a delay, not a loss.

For swaps, the mempool is where timing risk lives. A deposit that pays a fee rate well below current demand has zero network confirmations until a miner picks it up, and the order cannot move forward until the deposit confirms. Reading the mempool before you send tells you what fee rate a deposit needs to clear in the next block, the next half hour, or the next few hours. The guide to Bitcoin mempool and fees walks through how to read those estimates.

Suppose you send BTC to start a fixed-rate swap on Zest while the mempool holds twenty blocks' worth of transactions, and your wallet picked 4 sat/vB when the next-block rate is 30 sat/vB. The deposit waits unconfirmed, and the quote's deposit window can close before it lands. If your wallet supports Replace-by-Fee, bumping the fee is the direct fix. If it doesn't, the Bitcoin transaction accelerator compares the transaction against live mempool estimates and can rebroadcast it so more nodes and miners see it, though no tool can force a miner to include a low-fee transaction.

Ethereum and other account-based chains have mempools too, where the same idea applies to the gas fee a transaction bids.