← GlossaryWallet basics
What Is a Mempool? Where Unconfirmed Transactions Wait
The mempool (memory pool) is the set of valid, unconfirmed transactions a node keeps in memory while they wait to be included in a block; every node has its own.
Key takeaways
- A broadcast transaction sits in nodes' mempools until a miner includes it in a block.
- There is no single global mempool; each node keeps its own, and they usually look alike.
- Mempools are not permanent: unconfirmed transactions can be dropped when nodes restart or purge old entries.
- Higher fee rates generally get picked sooner when blocks are full.
What the mempool is
When you send a transaction, your wallet broadcasts it to nodes. Each node checks it against the rules and, if it is valid, keeps it in memory. That in-memory set is the mempool. Miners build candidate blocks from the transactions they can see, so the mempool is the waiting room between "sent" and "confirmed".
It is not one shared list
Every node maintains its own mempool, and the contents differ slightly depending on what each node has received and its own limits. Bitcoin's developer documentation notes that the pool is stored in non-persistent memory, so when a peer shuts down its pool is lost except for transactions its wallet holds. A transaction that never gets mined tends to fade from the network as peers restart or purge entries.
Why a transaction can wait
If more transactions are waiting than fit in the next blocks, miners tend to choose those paying higher fees for their size. A low-fee transaction may wait through several blocks, or be dropped. Some wallets let you replace a transaction with a higher-fee version while it is still unconfirmed, if both the wallet and the chain support it.
What to do when a payment seems stuck
Check the transaction on a block explorer. If it is listed as unconfirmed, it is still in some mempools. Wait, or use your wallet's fee-bumping option if it has one. Do not send the same payment again until you know the first one has been dropped, or you may pay twice.
Example
You send coins with a very low fee during a busy period. Your wallet shows 'pending'. A block explorer shows the transaction as unconfirmed, which means it is waiting in mempools. Over the next hours, blocks with higher-fee transactions fill first; yours is mined when space frees up, or it is eventually dropped.
Put it into practice
Send with a fee you can see
Scrypt Wallet lets you choose your preferred block explorer, so you can check whether a transaction you sent is still waiting to confirm.
Create a free wallet Get Scrypt for AndroidFrequently asked questions
Is the mempool the same on every node?
No. Each node has its own mempool, so they are similar but rarely identical.
Can an unconfirmed transaction disappear?
Yes. Because mempools live in memory, never-mined transactions tend to drop out as nodes restart or purge old entries.
Does a bigger mempool mean higher fees?
Often, because more transactions compete for the same block space, but the relationship depends on the chain and its block capacity.
Written by the Scrypt Wallet team · Updated 2026-10-02. General education, not financial, tax or legal advice.