Mempool
Also called: memory pool · pending transactions
Definition
The mempool is the waiting area where valid but unconfirmed transactions sit on network nodes until a miner or validator includes them in a block.
How mempool works
When you send a transaction, it is broadcast to nodes, which check it and store it in their mempool. Each node keeps its own version, so there is no single global mempool, but they are usually similar. Miners or validators then pick transactions from the pool to build the next block.
Block space is limited, so transactions usually compete on fees. On Bitcoin, miners tend to prioritize transactions with a higher fee per virtual byte. On Ethereum, users set a priority tip on top of the base fee. When the mempool is crowded, low-fee transactions can wait for hours, and some may eventually drop out of nodes' pools.
The mempool is also where MEV happens on chains with a public mempool: bots watch pending trades and try to front-run or sandwich them. Some wallets offer private transaction routes to avoid this. If your transaction is stuck, options include waiting, using replace-by-fee or a speed-up feature to pay more, or cancelling with a replacement transaction where your wallet supports it.
Example
Suppose the Bitcoin mempool is busy and transactions paying 20 sat/vB are being confirmed, while yours paid 5 sat/vB. Your transaction is 140 vB, so you paid 700 satoshis. To bump it using replace-by-fee to 22 sat/vB, the new fee would be 140 x 22 = 3,080 satoshis, an extra 2,380 satoshis, and it would likely confirm within the next few blocks if demand stays the same.
Common mistakes
- Sending a transaction with the lowest possible fee during congestion and expecting it to confirm quickly.
- Resending the same payment again while the first one is still pending, risking paying twice.
- Placing large DEX swaps through the public mempool with wide slippage, which invites sandwich attacks.
Learn and practise
FAQ
Why is my transaction stuck in the mempool?
Usually the fee is too low compared with other pending transactions. You can wait, or speed it up with a higher fee if your wallet supports replace-by-fee or a similar function.
Can a transaction disappear from the mempool?
Yes. Nodes can drop very low-fee transactions after a period or when the pool is full. The funds stay in your wallet if the transaction was never confirmed.