What it is
A mempool, short for memory pool, is a collection of unconfirmed transactions held temporarily by a Bitcoin node. When a user sends a transaction, it is broadcast to the network and first enters the mempool of various nodes. These transactions wait in the mempool until a miner selects them to be included in a new block. Each node maintains its own mempool, which can vary slightly in content and size due to propagation delays and different transaction acceptance policies.
The size and activity of the mempool directly influence transaction processing times and fees. When the mempool is congested with many transactions, miners prioritize those offering higher fee rates, leading to increased costs for users. News often reports on "mempool congestion" during periods of high network activity, such as market rallies or specific protocol events like Ordinals minting. Users track mempool data via block explorers to estimate optimal fee rates and transaction confirmation times.
Why it matters
Understanding the mempool helps retail investors grasp why transaction fees fluctuate and why their Bitcoin transfers might take longer during peak network usage.
Reviewed under editorial standardsUpdated September 26, 2026Not investment advice