What is the mempool?
- Author
- CRYPTO PORT Editorial
- Published
- Updated
- Reading time
- 5 min
In short
The mempool is the waiting room where each node holds unconfirmed transactions before they enter a block. Whoever builds the next block picks from it. That selection is why higher-fee transactions get through first when the network is busy.
Key points
- A node-local pool of transactions awaiting confirmation
- Contents differ per node; there is no single canonical mempool
- Block producers generally pick the highest-paying transactions
- Transactions left too long may be evicted
Definition
An in-memory set of transactions a node has received but that are not yet in a block, used as the candidate pool when building one.
A transaction leaves your wallet, reaches the node it connects to, and is relayed onward neighbour by neighbour. Each node validates it and, if it passes, keeps it in its own mempool. At this stage nothing is settled and no block has claimed it.
Crucially, every node's mempool is different. Propagation delays and each node's own minimum-fee and capacity settings mean one node routinely holds transactions another has never seen. The 'pending' count on an explorer is just what that provider's node can see.
Block producers must choose which transactions fit in limited space, and they generally fill from the highest fee rate down. When demand is high, cheap transactions wait through many blocks. On Bitcoin, once a node's mempool hits its size limit, the lowest-paying entries are evicted.
Watch out for
- · A transaction visible in the mempool is not settled; treating it as received exposes you to reversal
- · A transaction vanishing from view is not a safe cancel — another node may still relay and confirm it
- · Pending transactions are public, so large orders can be front-run before they confirm