What is Mempool (Memory Pool)?
Crypto Glossary Definition
The mempool is where pending Bitcoin transactions wait for their chance to be written to the blockchain. A mempool is hosted by a node. The order by which transactions are selected from the mempool is based on the transaction fee paid to the miners.
Why Mempool (Memory Pool) Matters
The mempool is the waiting area of unconfirmed transactions that have been broadcast to the network but not yet included in a block. A congested mempool is what drives transaction/gas fees up, since users effectively bid for miners or validators to prioritize their transaction.
Mempool (Memory Pool) in Practice
A user submits a Bitcoin transaction attaching a fairly low fee, hoping to save a few dollars, right as network activity happens to spike due to a popular ordinals inscription event flooding the chain with transactions. Their transaction lands in the mempool alongside tens of thousands of others, all waiting to be picked up by a miner and included in the next block. Because miners are economically motivated to prioritize whichever transactions pay the highest fee per byte of data, the user's cheap transaction sits unconfirmed for hours while transactions willing to pay a premium jump the queue ahead of it. Checking a mempool visualizer website, they can see the current backlog visually represented as colored blocks stacked by fee rate, with their own transaction sitting somewhere in the low-fee, low-priority zone. Frustrated, they use a fee-bumping technique like Replace-By-Fee, rebroadcasting the same transaction with a higher fee attached to jump ahead in the mempool's effective queue. Within the next block or two, it finally confirms. This scenario illustrates why mempool congestion is the direct mechanical cause behind fee spikes during periods of high demand — it isn't arbitrary, but a real-time auction where transactions with the least urgency, or lowest fee, wait the longest for space in the next block.
Still have questions about Mempool (Memory Pool)?
Ask ARIA, our free AI crypto intelligence agent, for a deeper explanation.
Ask ARIA →