The mempool is the holding area where unconfirmed transactionsTransactionA record of crypto moving from one wallet address to another, signed with the sender's private key and stored permanently on the blockchain. wait to be included in a block. The name is short for memory pool, because the transactions are held in the computer's working memory. When you send bitcoin, the transaction first goes into the mempool and waits there until a miner includes it in a new block. Ethereum and most other networks have a similar holding area.
Every node has its own
There is no single shared mempool. Each NodeNodeA computer that runs a crypto network's software, keeps a copy of the blockchain and checks that new transactions and blocks follow the rules. keeps its own list of transactions it has heard about and accepted. The lists are usually very similar but not identical, because transactions spread across the network at different speeds and nodes can have different settings.
In the Bitcoin Core software, the mempool is limited to 300 MB by default. A transaction is dropped automatically if it has waited two weeks without getting into a block. The coins then never left, and they are still with the sender.
Why do fees rise?
Space in each block is limited. Miners usually pick the transactions that pay the highest network feeNetwork feeA fee paid to a crypto network, not to a bank or company, for including your transaction in a block. relative to their size. When many people send at once, the queue grows and people bid higher to get in sooner. Transactions with low fees can then sit stuck for hours or days.
When a node's mempool fills up, it throws out the transactions paying the lowest fees.
What to keep in mind
- Many block explorersBlock explorerA website where anyone can look up transactions, wallet addresses and blocks on a blockchain. show the state of the mempool. There you can see how busy the network is before you choose a fee.
- Some wallets let you raise the fee on a stuck transaction. This is called replace-by-fee (RBF).
- A transaction waiting in the mempool is not confirmed. It gets its first ConfirmationConfirmationA sign that a transaction has been included in a block; each new block added on top counts as one more confirmation and makes it more secure. once it makes it into a block.
Everything in Myntkaup Learn is written and regularly reviewed by the Myntkaup editorial team. It is for education and is not investment advice. Editorial policy. Updated 18 October 2026.