The mempool, short for memory pool, is the waiting area where transactions sit after being broadcast to a network but before being included in a block. Every node keeps its own copy in memory.
Think of it as a queue with no fixed order. From it, miners and validators pick, and what they pick is mostly determined by what you paid.
How a transaction gets through it
Five stages, and the mempool is stage two.
- You sign and broadcast a transaction from your wallet.
- Nodes validate it, check it is not a double spend, and hold it in their mempool.
- A miner or validator selects transactions to include in the next block, generally favouring the highest fee per unit of data.
- The block is mined or proposed, and your transaction leaves the mempool.
- Subsequent blocks build on top, and confirmations accumulate.
Sitting in the mempool, a transaction has happened in no meaningful sense. It is a request. Until it lands in a block, it can be replaced, dropped, or simply wait.
The fee market
Because block space is finite, the mempool matters at all.
Bitcoin blocks hold roughly 4 million weight units. Ethereum blocks have a gas limit. Arriving in greater numbers than fit, transactions offering more per byte get selected first. Everyone else waits.
Fee rates are quoted per unit of size rather than as a total. On Bitcoin that is satoshis per virtual byte. Paying a high total fee does not help a large transaction beat a small one paying a higher rate.
Congestion therefore has a signature. Size grows, fee rates climb, and low-fee transactions accumulate at the back.
What happens if your transaction gets stuck
Three options, depending on the network and what you set.
Wait. During quiet periods fee rates fall, and a stuck transaction can confirm hours later without intervention.
Replace-by-fee. Defined in BIP-125, RBF lets you rebroadcast the same transaction with a higher fee, replacing the original. For it to work, your wallet must have flagged the transaction as replaceable when you sent it.
Child pays for parent. Spend the unconfirmed output in a new transaction carrying a high fee. To claim the child, miners must include the parent, so both confirm together.
Transactions can also be dropped entirely. By default, most Bitcoin nodes cap their mempool at 300 MB and evict the lowest-fee transactions when full. Never confirmed means never spent, so the funds remain yours.
Notable congestion events
Two illustrate different causes.
In 2021, a power outage in Xinjiang removed roughly 24% of Bitcoin's global hashrate almost immediately. Blocks slowed. Unconfirmed transactions accumulated sharply, because the supply of block space fell while demand held.
Through 2023 and 2024, Ordinals and BRC-20 tokens pushed large volumes of data into Bitcoin blocks, competing directly with ordinary payments. Demand shock rather than supply shock, and fee rates rose accordingly.
Both show the same thing from opposite directions. Depth is a ratio, not an absolute.
The mempool as a public data source
Because everything waiting is visible, there are consequences.
To time transactions, traders watch depth and fee rates. Analysts read large pending transfers as signals. MEV searchers watch continuously, looking for transactions worth reordering, front-running, or sandwiching.
On Ethereum this created a market. Around the public mempool, private order flow now routes transactions entirely, through relays like Flashbots, specifically so bots cannot see them before they settle. A substantial share of Ethereum transactions never appear publicly pending at all.
What the mempool does not tell you
- No single global queue exists. Each node holds its own view, and they differ. This also affects what a block explorer shows you. A transaction visible to one node may not have reached another.
- Depth does not predict fees reliably. A large mempool full of low-fee transactions may clear slowly without raising the rate you need.
- Pending means nothing is settled. Treating an unconfirmed transaction as final is how double-spend acceptance happens, whether you are moving funds to a crypto address or receiving them.
- It cannot tell you order. Fee rate is the main influence on selection, not a guarantee of it.
Where mb.io fits
Mempool mechanics matter when you move funds on-chain. Trades inside a centralized exchange never touch it, because they settle in the platform's own ledger.
mb.io is a regulated crypto spot exchange backed by MultiBank Group, a financial institution founded in 2005 that serves more than 2 million clients across 100+ countries.
- Regulated by VARA in the UAE and AUSTRAC in Australia
- 40-nanosecond execution speed
- 10/10 security score from Hacken, an independent blockchain security auditor
- Institutional-grade MPC custody powered by Fireblocks, with segregated client funds
- Buy, sell, and swap in three steps, from sign-up to purchase
- 24/7 customer support, on web and on the iOS and Android apps
Open your account and start trading on mb.io.
Frequently asked questions
What does mempool stand for?
Memory pool. It is the set of valid but unconfirmed transactions a node holds in memory while they wait to be included in a block.
Why is my transaction stuck in the mempool?
Your fee rate was too low relative to competing transactions. Options: wait for congestion to ease, use replace-by-fee if your wallet enabled it, or use child pays for parent.
Can a transaction be cancelled while in the mempool?
Not cancelled, but potentially replaced. Replace-by-fee lets you broadcast a competing version with a higher fee, which can redirect funds if the original has not confirmed. It only works if the transaction was flagged replaceable.
Can a mempool transaction be dropped?
Yes. By default, most Bitcoin nodes cap the mempool at 300 MB and evict the lowest-fee transactions when it fills. Dropped means never confirmed, so the funds were never spent.
Is the mempool the same on every node?
No. Each node maintains its own view based on what it has received and its own policies, so mempools differ between nodes.
How do I check the mempool?
Public explorers display current size, fee rate estimates, and pending transaction counts. They show one node's view rather than a definitive global state.
What is MEV and how does the mempool relate?
Maximal extractable value, where bots profit by reordering, inserting, or front-running pending transactions. In the public mempool they find those opportunities, which is why private order flow routing exists on Ethereum.
How many confirmations should I wait for?
That depends on the amount and the network. Sitting in the mempool means zero confirmations, which is not final. Larger transfers conventionally wait for more blocks to build on top.
