Mempool

blockchain · node

A node’s local collection of valid transactions waiting to be included in a block.

Also known as: memory pool

1.general

A mempool is a node’s waiting area for transactions it has accepted but that are not yet included in a block. There is no single shared mempool: nodes can see different transactions or apply different acceptance rules.

2.differences

Nodes can see different transactions because they connected at different times, use different policies, have different ancestors, or evicted entries under memory pressure. A transaction can be valid for a block yet absent from a particular mempool, and a policy-accepted transaction can later conflict or become invalid after a reorganization.

3.selection

Miners commonly choose transactions by fee rate while accounting for parents, children, packages, signature-check and size limits, and local policy. When a block is accepted, nodes remove included transactions and any now-conflicting spends; other entries remain pending, expire, or are evicted.

4.measurement

A mempool dashboard is a snapshot of one node or provider. Counts, total bytes, fee bands, and waiting-time estimates can help describe pressure, but they do not guarantee another node's view or a transaction's confirmation time.

Read how blockchain congestion builds to connect pending transactions with demand and fees. Blockspace Lab lets you explore those relationships in a simulation.

Technical reference: Bitcoin developer guide to transaction broadcasting.