Mempool
The mempool is the waiting room for transactions: valid, broadcast, but not yet in a block.
Photo: “Waiting Room Chairs Dying Plant Door” by cdsessums, CC BY 2.0, via Flickr (edited: cropped and resized).
The mempool, short for memory pool, is where a node keeps unconfirmed transactions after receiving them; miners fill new blocks from their own node’s mempool [1]. There is no single global mempool — each node has its own similar but separate set [1].
Key points
- 1Every transaction passes through mempools before its first confirmation.
- 2Each node keeps its own mempool under its own policy settings.
- 3Miners generally pick the transactions paying the most per byte first.
What does the mempool do?#
Full nodes keep track of unconfirmed transactions that could go into the next block. That is essential for miners, who build blocks from them, and useful for anyone who wants to see pending payments [2]. When a transaction is mined, nodes remove it from their mempools; if it is never mined, it can eventually be evicted or expire.
| Step | Value |
|---|---|
| Tx A: 3,000 sats for 200 vB | 15 sat/vB — first |
| Tx B: 8,000 sats for 1,000 vB | 8 sat/vB — second |
| Tx C: 450 sats for 150 vB | 3 sat/vB — last |
Tx B pays the most in total but less per unit of space. The developer guide describes miners filling blocks with the highest fee-per-byte transactions first [3].
Why is there no single mempool?#
Each node hears about transactions at slightly different times and applies its own rules about which to keep. The Bitcoin Wiki calls the idea of one network-wide mempool a common misconception, and notes that if every node were guaranteed the same mempool, there would be no need for mining at all [1]. Ethereum works the same way: each execution client adds valid transactions to its local mempool and gossips them to other nodes [4].
Which words are related?#
- Confirmation — what a transaction gets when it leaves the mempool in a block.
- Node — each keeps its own mempool.
- Transaction fees explained — how fee rates are set.
Frequently asked questions#
Where does the name come from?
Old Bitcoin versions held it only in memory and wiped it on restart, hence “memory pool” [1].
Does a transaction in the mempool count as paid?
Not yet. It has zero confirmations and can still be replaced or dropped. Wait for at least one confirmation unless you trust the sender.
What happens when a mempool is full?
The node trims it, evicting the lowest fee-per-byte transactions first [1].
Sources#
Grade A = primary source (regulator, protocol specification, client code, original author). Grade B = expert secondary source used for explanation only.
- BBitcoin Wiki. Vocabulary (Memory pool, Confirmation, Hash function, Node), 2026.
- Abitcoin.org developer documentation. Developer Guide: P2P Network, 2026.
- Abitcoin.org developer documentation. Developer Guide: Transactions, 2026.
- Aethereum.org. Proof-of-stake (PoS), 2026.