The mempool (short for “memory pool”) is a sort of digital “waiting room” for all transactions that have been sent to the Bitcoin network but have not yet been included in a block. Each full node in the network has its own mempool where it temporarily stores these unconfirmed operations.
Analogy: Airport Waiting Room
To understand how the mempool works, imagine a waiting room before boarding a plane (block).
- Passengers are transactions that want to “fly”.
- The plane (block) has a limited number of seats (limited block size).
- Airline staff (miners) decide which passengers to let on board the next flight.
All passengers who didn’t fit on the current plane remain in the waiting room (mempool) and wait for the next flight.
How do miners select transactions from the mempool?
Miners are economically motivated to maximize their profit. And their profit, as we know from the article on block rewards, consists of subsidies and transaction fees.
Therefore, when forming a new block, a miner will act like that airline employee who prioritizes boarding business class passengers who paid the most for their ticket. They will sort all transactions in the mempool by the size of their fee (usually measured in satoshis per byte, sat/vB) and begin filling the block with the most “expensive” ones.
If you sent a transaction with a very low fee, it will end up at the very end of this “queue”. If the network is not congested, it might be included in the very next block. But if there are many bidders, your transaction might “get stuck” in the mempool for several hours or even days until its turn comes.
What is “mempool congestion”?
During periods of high market activity (for example, during a sharp rise or fall in Bitcoin’s price), the number of transactions sent to the network increases sharply. The mempool begins to “bloat” because more transactions are coming in than miners can include in blocks (which are consistently produced every 10 minutes).
At this point, a real “fee war” begins. Users who want their transaction to go through quickly start setting higher and higher fees to “outbid” others. This leads to a sharp increase in the average transaction cost on the network.
Transactions with low fees can remain in the mempool for a very long time in such a situation. However, they don’t disappear forever. Most nodes are configured to “forget” about old unconfirmed transactions after a certain period (usually 2 weeks), and the coins simply “return” to the sender’s wallet.
Thus, the mempool is a dynamic and competitive market where users compete for limited space on the blockchain. The size of the mempool and the average fee within it are excellent indicators of the current congestion and “health” of the Bitcoin network.
