Mempool

💡 The Plain-English Definition

The mempool — short for memory pool — is the waiting room where unconfirmed Bitcoin transactions sit until a miner includes them in a block. Every full node (a computer independently running and validating the Bitcoin blockchain) maintains its own mempool, and when blocks fill up, competition for space drives fees higher.

🤔 But Why Though?

When you broadcast a Bitcoin transaction, it doesn’t go directly into a block — it enters the mempool first, where it waits to be picked up by a miner. Miners scan the mempool and select transactions to include in the next block they’re building. Since each block has a fixed size limit (approximately 4 million weight units of transaction data), and since thousands of transactions may be waiting, miners rationally choose the transactions with the highest feerate (fee per unit of transaction size — measured in satoshis per virtual byte) to maximise their revenue per block.

A key detail: every full node maintains its own independent mempool. There is no single global mempool — it’s a distributed set of overlapping lists, each node holding the transactions it has seen and considers valid according to its own policy. Most nodes use Bitcoin Core’s default settings and mempools largely overlap, but they’re never identical. This matters when a transaction is in some mempools but not others, or when low-feerate transactions get dropped by nodes with stricter minimum fee policies. Congestion dynamics are self-explanatory once you understand the structure: when many transactions compete for limited block space, users must bid higher feerates to ensure prompt confirmation. This is why fees spike during bull markets (everyone transacting simultaneously), after Ordinals or Runes activity (on-chain data consuming block space), or during exchange outflows (everyone withdrawing at once). When the mempool is quiet, even low-feerate transactions confirm quickly because miners have spare block space to fill. The concept of a “pending” transaction is just a transaction in the mempool waiting its turn — not broken, not lost, just waiting for a miner to include it.

🌍 The Real-World Analogy

The mempool is like the queue at a busy post office on the last posting day before Christmas. Hundreds of packages wait. The staff (miners) process packages in order of priority — but unlike a fair queue, priority is determined by how much extra you paid for express delivery (feerate). The person who paid for overnight shipping gets processed first regardless of when they arrived. Someone who paid for standard delivery might wait for hours. And if the queue gets too long, the post office stops accepting new packages below a minimum postage amount.