What is the mempool?
Pressing send does not put a payment on the blockchain. It puts it in a queue — and the queue has no single location.
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The mempool (memory pool) is where a blockchain node keeps transactions it has received but that are not yet in a block. Each node keeps its own copy. Miners or validators pick transactions from it to build the next block, generally favouring those that pay higher fees.
How does the mempool work?
When you send a transaction, your wallet broadcasts it to the network. Ethereum's docs describe it being added to a pool of all other pending transactions until a validator includes it in a block. Bitcoin's developer guide says full nodes may keep track of unconfirmed transactions eligible for the next block, relaying each new one to their peers.
NIST stresses that the pending-transaction pool is maintained locally by each node, not held centrally — so there is no single, official mempool, and two nodes can see slightly different queues. More on the flow in how crypto transactions work and on nodes in what is a node.
Which transactions leave the mempool first?
Bitcoin's developer guide explains that miners rank transactions by fee per byte, highest first. Size matters as much as the total fee.
On Ethereum the equivalent lever is the per-gas price; see gas fee.
What happens to a transaction stuck in the mempool?
It waits. Bitcoin's guide notes that the memory pool is not saved permanently: when a node shuts down it loses its pool, apart from its own wallet's transactions, so an unconfirmed transaction that nobody re-broadcasts can gradually disappear from the network. Transactions in a block that is later dropped from the chain can return to the pool. Until a transaction is in a block it has not happened — which is why a pending payment should not be treated as received. You can watch its status on a block explorer.
Questions readers ask
Is the mempool public?
Pending transactions are broadcast openly between nodes, so anyone running a node, or using a service that shows its data, can see them before they are confirmed.
Why is my transaction pending for so long?
Usually because its fee rate is lower than what others are paying, so it keeps being passed over. Confirmation speeds up when demand falls or when the fee is raised, where the wallet allows it.
The mempool is a queue, not a ledger. Each node keeps its own, fee rate decides the order, and nothing in it is final until it is mined or validated into a block. Mining itself is explained in how Bitcoin mining works.
Sources
- Bitcoin Project (developer.bitcoin.org), P2P network — memory pool (Bitcoin developer guide) (2026)Primary source
- Bitcoin Project (developer.bitcoin.org), Transactions (Bitcoin developer guide) (2026)Primary source
- National Institute of Standards and Technology, NISTIR 8202: Blockchain Technology Overview (2018)Primary source
- ethereum.org, Transactions (developer documentation) (2026)Primary source
Educational content only — not financial, investment, legal or tax advice. Crypto-assets are high-risk and you could lose all the money you put in. Rules differ by country; check with your national regulator. See our risk disclosure and editorial policy.
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