Mempool
In plain English
The waiting room for transactions: after you hit send, your transaction sits in the mempool until a miner or validator picks it up and puts it in a block.
What is the mempool?
The mempool (short for "memory pool") is the set of transactions that have been broadcast to a blockchain network but have not yet been included in a block. Every node keeps its own copy in memory, which is why there is no single official mempool — nodes can hold slightly different sets.
How it works
When you send a transaction, your wallet broadcasts it to nearby nodes, which relay it across the network. Each node validates the basics — correct signature, sufficient balance, no double-spend — and if it passes, holds the transaction in its mempool. Miners (Bitcoin) or validators (Ethereum and other proof-of-stake chains) then select transactions from that pool to build the next block.
Why it matters for fees
Block space is limited, so when more transactions arrive than a block can hold, the mempool grows and competition begins. Transactions offering a higher fee get picked first, and lower-fee transactions wait. This is why fees spike during busy periods: you are bidding for a scarce slot, not paying a fixed price.
A crowded mempool is a useful signal. If you check a mempool explorer and see a large backlog, expect slow confirmations unless you raise your fee. When the backlog clears, fees usually fall.
What happens to stuck transactions
A transaction that pays too little may sit unconfirmed for hours. Nodes eventually drop it — typically after around two weeks on Bitcoin — and the funds return to your control, since they never actually moved. Many wallets let you speed things up beforehand using replace-by-fee (RBF), which rebroadcasts the same transaction with a higher fee.