What is a Mempool?
A mempool (short for ‘memory pool’) is a temporary storage space on a blockchain network where pending transactions wait to be confirmed by those responsible for transaction finality, typically known as miners or validators.
Mempools function as a queue for unconfirmed transactions before they are included in a new block. The mempool is an essential part of how blockchain networks process and manage transactions.
How does a Mempool Work?
Mempools receive transactions once a user sends funds from their wallet address. They then wait to be processed by miners (on Bitcoin) or validators (on popular Proof-of-Stake (PoS) blockchain networks such as Ethereum).
Depending on network load, the size of the mempool can fluctuate significantly as transactions with higher fees receive faster processing.
Mempools are not identical across blockchains — each network node maintains its own version of the mempool and may harbor different transactions.
How Long Does a Mempool Transaction Take?
Public blockchains allow for anyone to view the size of the mempool at a given time, and gauge how large the fee should be for a transaction to be processed within a given number of blocks.
Bitcoin has various dedicated online resources for mempool analysis, along with fee information. During volatile market conditions or when other phenomena cause unusually high numbers of unspent transaction outputs (UTXOs), the mempool can expand and transactions — especially those with a low fee — can stay unconfirmed for a significant length of time. This is commonly referred to as mempool ‘bloat’.
In some cases, a user can increase the fee post factum in order to make their transaction more appealing to miners or validators.
