Mempool
Also: memory pool
network · intermediate
The set of valid transactions a node has seen but not yet included in a block.
Each Bitcoin [node](/glossary/node) keeps its own mempool — a buffer of transactions it has validated and is willing to relay, but which haven't been mined into a [block](/glossary/block) yet. There is no single canonical mempool; nodes diverge based on their policies, relay timing, and resource limits.
When a wallet broadcasts a transaction it propagates through the peer network, landing in mempools across the globe. Miners select from their own mempool — usually [fee rate](/glossary/fee-rate) descending — to build the next block. Transactions with low fees may sit there indefinitely; most node implementations evict the lowest-fee entries after about two weeks.
The mempool is the price-discovery layer for block space. Watching its depth is the easiest way to see Bitcoin's congestion in real time — the [fee-market tool](/tools/fee-market) renders this directly.