What is a Mempool?
A mempool is the waiting area where nodes hold transactions that have been broadcast but not yet included in a block. When the network is busy the mempool grows, and fees largely decide which transactions confirm first.
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A mempool (short for memory pool) is the waiting area where nodes hold transactions that have been broadcast to the network but not yet included in a block. When you send bitcoin, your transaction first lands in the mempool and sits there until a miner picks it up. When the network is busy the mempool swells, and the fee you paid largely decides how quickly you get confirmed.
How does the mempool work?
A transaction’s path from your wallet to a block looks roughly like this:
- Your wallet sends the signed transaction to a node.
- The node checks it: is the signature valid, have the coins already been spent, does the fee meet the minimum?
- If it passes, the transaction is added to that node’s mempool and relayed to other nodes.
- Miners choose the most profitable transactions from their mempool to build a candidate block.
- Once the transaction is in a block, it leaves the mempool and gets its first confirmation.
There is no single, global mempool. Every node keeps its own, receives transactions at slightly different times and may apply different policies. The pending list you see on a block explorer reflects the nodes that service is connected to.
Why is my transaction stuck in the mempool?
Block space is scarce. Bitcoin produces a block roughly every ten minutes, and each block holds a limited amount of data. When demand rises, users compete for space with fees. Miners prioritize transactions that pay the most per virtual byte of data, a rate shown as sat/vB (satoshis per virtual byte).
Fees depend on a transaction’s data size, not on the amount being sent. A $10 transfer and a $1 million transfer of the same size can pay the same fee. If you paid a low rate, newer transactions with higher fees can jump ahead of you. That doesn’t mean your coins are lost; your transaction is simply still in the queue.
How do you speed up a stuck transaction?
- RBF (Replace-by-Fee): you broadcast a new version of the transaction that spends the same inputs with a higher fee, replacing the original. A “bump fee” button in your wallet typically does this.
- CPFP (Child Pays for Parent): you spend the incoming, unconfirmed coins in a second transaction with a high fee. To collect that fee, a miner has to include the parent transaction as well.
If the stuck transaction is an exchange withdrawal, the exchange controls it, so you’ll need to contact its support team rather than bumping it yourself.
What happens when a transaction drops out of the mempool?
Nodes cap the memory their mempool can use. Bitcoin Core’s default limit is 300 MB, and transactions that linger for about two weeks are evicted by default. When the mempool fills up, the lowest-fee transactions are dropped first. A dropped transaction is usually treated as if it never happened, and the coins show as spendable again, but other nodes may still hold a copy. Before paying again from the same balance, check the transaction’s status on an explorer.
Ethereum has a mempool too, but fees there consist of a base fee and a priority tip under EIP-1559. Because some actors profit by reordering transactions, a practice known as MEV, many users now send transactions through private channels instead of the public mempool. Our gas fee guide explains Ethereum fees in detail.
Frequently asked questions
Can I cancel a transaction in the mempool?
There is no cancel button as such. On Bitcoin you can replace it with a higher-fee RBF transaction that sends the same coins back to yourself; on Ethereum you send a new transaction with the same nonce.
Where can I see how busy the mempool is?
For Bitcoin, explorers such as mempool.space show the number of pending transactions and suggested fee rates in real time.