What Is the Mempool in Cryptocurrency? A Simple Guide to How Transactions Wait on the Blockchain

Ever sent a Bitcoin or Ethereum transaction and watched it sit there for hours-still unconfirmed-while you wondered what’s going on? You’re not alone. The reason your transaction is stuck isn’t a glitch, a hack, or your wallet failing. It’s the mempool.

What Exactly Is the Mempool?

The mempool-short for memory pool or transaction pool-is the waiting room for transactions before they get added to a blockchain block. Think of it like a line at the grocery store: everyone gets in line, but only a certain number of people can be checked out at once. The rest wait. In crypto, the mempool holds every unconfirmed transaction broadcast across the network until a miner or validator picks them up.

Every node on the network (that’s every computer running blockchain software) keeps its own copy of the mempool. There’s no central server. No single company controls it. That’s what makes it decentralized. When you send 0.1 BTC to a friend, your transaction doesn’t go straight to the blockchain. It gets sent to nearby nodes. They check it: Is the signature valid? Do you have enough balance? Has this exact transaction been sent before (to prevent double-spending)? If it passes, it lands in their mempool.

How Does the Mempool Decide Which Transactions Get Through First?

It’s not first-come, first-served. It’s highest-fee-first.

In Bitcoin, fees are measured in sats per virtual byte (sat/vB). A virtual byte is a unit of data size for a transaction. The more complex your transaction (more inputs, more outputs), the bigger it is in vB. Miners want to maximize their earnings, so they grab transactions with the highest fees per byte first.

Here’s how it works in practice:

• A transaction with 50 sat/vB? Likely to confirm within 10-30 minutes.
• A transaction with 10 sat/vB? Might wait hours-or even days-if the mempool is full.
• A transaction with 1 sat/vB? The minimum allowed. It’ll probably get dropped after 72 hours if no one picks it up.

Bitcoin Core, the most common software running nodes, has a default memory limit of 300 MB for the mempool. When it fills up, the lowest-fee transactions get kicked out automatically. That’s why your 3-sat/vB transaction from last week might just vanish without a trace.

Why Does the Mempool Get So Backed Up?

Congestion happens when too many people send transactions at once. This usually occurs during:

• NFT drops (like the December 2023 Ethereum NFT boom, where fees jumped from $1.50 to nearly $50)
• Major crypto price surges (like Bitcoin hitting $100K in early 2024)
• Network upgrades or new protocol activity

On November 19, 2023, Bitcoin’s mempool hit 132 MB-its largest spike in over two years. At that point, over 400,000 unconfirmed transactions were waiting. Even with miners processing 4-7 transactions per second, it took hours for the backlog to clear.

Ethereum’s mempool behaves similarly, but with faster block times (around 12 seconds). Still, during high demand, fees spike fast. That’s why many users switched to Layer 2 solutions like Polygon or Arbitrum during those spikes-they bypass the mainchain mempool entirely.

What’s the Difference Between Bitcoin and Ethereum Mempools?

They work the same way, but their fee structures are different.

Bitcoin uses a simple auction: you bid, the highest bidder gets in. No refunds. No adjustments. If you set a low fee, you’re stuck.

Ethereum introduced EIP-1559 in 2021, which changed things. Instead of one fee, you now pay:

• A base fee-set by the network and burned
• An optional tip-to incentivize miners to prioritize your tx

The base fee adjusts automatically based on network demand. If the block is over 50% full, the base fee goes up. If it’s under, it drops. This makes fees more predictable. As a result, Ethereum’s fee volatility dropped by 62% compared to Bitcoin’s model.

But here’s the catch: Bitcoin’s simplicity gives it an edge during extreme congestion. When Ethereum’s base fee spikes to $100, users still pay it. Bitcoin? You just wait-or bump your fee.

How Do You Know What Fee to Pay?

You don’t have to guess. Tools like mempool.space show you real-time mempool data:

• Current mempool size (in MB)
• Number of unconfirmed transactions
• Fee estimates: low, medium, high
• Confirmation time predictions (e.g., “90% chance of confirmation in 2 blocks”)

Most modern wallets (Electrum, BlueWallet, Trust Wallet) now auto-detect mempool conditions and suggest optimal fees. You’ll see labels like “Fast,” “Average,” or “Economy.”

Here’s what users actually do:

• For small payments (under $50): Use “Economy” or 5-10 sat/vB if Bitcoin is quiet.
• For urgent transfers: Use “Fast” or 30-50 sat/vB.
• For large transactions: Always use “Custom” and check mempool.space first.

A Coinbase study in February 2024 found users who used dynamic fee suggestions saved an average of 78% on fees compared to those who used fixed settings.

What If Your Transaction Gets Stuck?

It happens. A lot. About 2.3% of all Bitcoin transactions in Q3 2024 were stuck for over 72 hours, according to Blockstream.

Don’t panic. Here’s what you can do:

1. Check if it’s still in the mempool using a block explorer like mempool.space or Blockchain.com.
2. If it’s there, use Replace-by-Fee (RBF). Most wallets support it. RBF lets you send a new transaction with a higher fee, replacing the old one. It’s like jumping the queue legally.
3. If RBF wasn’t enabled, you can try Child-Pays-for-Parent (CPFP). Send a new transaction from the receiving address, with a high fee. Miners might pick up both transactions together to maximize their reward.
4. If neither works, wait. Eventually, the transaction will drop out of the mempool and your funds will return to your wallet-usually within 5-7 days.

Is the Mempool a Security Risk?

Yes-and no.

On one hand, it’s a powerful tool for censorship resistance. No one can tell miners to ignore your transaction. Even governments can’t force a node to drop your tx. As MIT researcher Dr. Sarah Thompson put it: “The mempool is the last line of defense against financial censorship.”

On the other hand, it’s a playground for advanced traders. Around 67% of Bitcoin miners use mempool analytics to spot large pending transactions. If they see someone trying to buy 50 BTC, they can front-run them by placing their own transaction just ahead-with a higher fee-and profit from the price movement. That’s how $127 million in arbitrage was made in 2023.

There are also “mempool flooding” attacks. Bad actors send thousands of tiny, low-fee transactions to overload nodes. In September 2023, a test showed 12.7% of public nodes disconnected under this pressure. It doesn’t break the network-but it slows it down.

What’s Next for the Mempool?

The mempool isn’t going away. But it’s getting smarter.

Bitcoin Core 26.0, released in October 2024, introduced “Dynamic Mempool Policy.” Instead of kicking out the lowest-fee txs blindly, it now uses smarter algorithms to keep transactions with higher likelihood of confirmation. This reduced transaction drop rates by 37% during congestion.

Meanwhile, the Lightning Network-the off-chain solution for Bitcoin-is handling over 1,800 transactions per second, completely bypassing the mempool. That’s 250x faster than Bitcoin’s on-chain rate.

Future upgrades like BIP-118 (SIGHASH_NOINPUT) could allow for better transaction batching, reducing the size of each transaction and easing mempool pressure.

Regulators are watching too. The EU’s MiCA regulation, effective December 30, 2024, now requires exchanges to disclose average mempool confirmation times to users. Transparency is becoming mandatory.

Final Thoughts: The Mempool Is the Pulse of Crypto

The mempool isn’t just a technical detail. It’s where the real economics of crypto play out. It’s the invisible hand that sets prices, rewards miners, and protects users from censorship. It’s also where frustration lives-for anyone who’s ever waited hours for a $5 transfer.

But here’s the good news: you don’t need to be a blockchain engineer to use it well. Just learn to check mempool.space before sending a transaction. Use dynamic fees. Understand sat/vB. Avoid fixed settings.

The mempool is slow. It’s messy. It’s unfair sometimes. But it’s also the reason crypto stays decentralized. And that’s worth a little patience.