Topic
What is a mempool?
Last reviewed:
- Quick fact 1
Nodes keep unconfirmed transactions in a memory pool until they are added to a block.
- Quick fact 2
Miners generally choose transactions that pay higher fees first.
- Quick fact 3
A transaction is confirmed once it is included in a block.
The short lesson
When you send crypto, your transaction does not go into the blockchain right away. First it is shared with the network’s computers, which keep it in a waiting area called the mempool, short for memory pool.
Each node keeps its own mempool. Miners or validators pick transactions from it to fill the next block. Space in a block is limited, so they usually pick the ones that pay higher fees first.
What this means for you:
- Busy times mean longer waits or higher fees.
- A low-fee transaction can sit in the mempool for a long time, and sometimes drops out.
- “Pending” means waiting in the mempool. It is not final until it is in a block.
Never pay a stranger to “unstick” a pending transaction. Check its status yourself on a blockchain explorer.
Related terms
Go deeper in Lesson 6: What a network fee is
Common questions
Is there one mempool?
No. Each node keeps its own, so they can differ a little.
Why is my transaction stuck?
Its fee may be low compared with others waiting. Some wallets let you raise the fee.
Can a pending transaction be cancelled?
Sometimes, by replacing it with a new one, depending on the wallet and network. Once confirmed, no.
Is the mempool private?
No. Pending transactions are generally visible to the network.