Quick Answer
The mempool (memory pool) is a waiting room for unconfirmed transactions. When you send crypto, your transaction doesn’t go directly into a blockchain block — it first enters the mempool, where it waits until a miner or validator picks it up. Transactions with higher fees get confirmed faster.
What is the mempool?
The word mempool comes from memory pool. It is a temporary storage area — held in the RAM of each node in the network — where valid but unconfirmed transactions wait to be included in the next block.
Every blockchain that uses a block-based structure has a mempool. Bitcoin and Ethereum each have their own mempool. There is not just one global mempool — each node maintains its own local copy, though they largely overlap.
Simple analogy: Think of the mempool as the queue outside a nightclub. The bouncer (miner) can only let in a limited number of people per round (block). If you pay more (higher fee), you move to the front. If the queue is too long and you paid too little, you might wait for hours — or eventually be turned away.
How does the mempool work?
Step 1 — You broadcast a transaction
You initiate a transfer from your wallet. Your wallet signs the transaction with your private key and broadcasts it to the network. The transaction is not yet confirmed — it has no block.
Step 2 — Nodes validate and accept the transaction
Each node that receives the transaction checks: Is the signature valid? Does the sender have enough funds? Does it follow the network rules? If everything checks out, the node adds the transaction to its local mempool and forwards it to neighbouring nodes.
Step 3 — The transaction waits in the mempool
The transaction sits in the mempool alongside thousands of others. Each node sorts them — typically by fee per byte (Bitcoin) or gas price (Ethereum). Higher-fee transactions sit at the top of the priority list.
Step 4 — A miner or validator picks up the transaction
When a miner (on Proof of Work networks) or validator (on Proof of Stake) builds the next block, they select transactions from the mempool — typically prioritising the highest fees. They fill the block up to the maximum size limit.
Step 5 — Transaction confirmed, removed from mempool
Once the block is validated and added to the blockchain, all transactions in that block are removed from the mempool across the network. Your transaction now has its first confirmation.
Mempool size and transaction fees
The mempool is the direct reason why transaction fees fluctuate. Understanding this connection helps you time transactions better and save money.
📈 Mempool congested
Thousands of transactions waiting. Miners can only include so many per block. To get confirmed quickly, you must outbid others. Gas fees spike. Bitcoin fees spike. Confirmation times stretch from minutes to hours.
📉 Mempool empty
Few transactions waiting. Miners include everything quickly. Even the minimum fee gets confirmed in the next block. Best time to send transactions — typically weekends or low-activity periods.
📊 How miners prioritise transactions
Bitcoin: Transactions are ranked by sat/vByte (satoshis per virtual byte). The more you pay per byte of data, the higher your priority. A simple transfer uses ~250 bytes; a complex one more.
Ethereum: Since EIP-1559, the fee has two parts — a mandatory base fee (burned, not paid to miners) and an optional priority fee (tip to the validator). A higher tip = faster confirmation.
Mempool on Bitcoin vs. Ethereum
| ₿ Bitcoin Mempool | 🔷 Ethereum Mempool (txpool) | |
|---|---|---|
| Also called | Mempool | Txpool (transaction pool) |
| Fee unit | sat/vByte | Gwei (base fee + priority fee) |
| Block time | ~10 minutes | ~12 seconds |
| Block size | ~4 MB (SegWit) | Variable gas limit (~15M gas target) |
| Congestion cause | High on-chain demand, halving events | NFT mints, DeFi events, token launches |
| RBF (replace-by-fee) | Supported — you can bump your fee | Supported — resubmit with higher gas |
What happens to stuck or dropped transactions?
If your fee was too low and the mempool is congested, your transaction may get stuck for a long time — or eventually dropped entirely.
Option 1 — Replace-by-Fee (RBF)
If you used a wallet that supports RBF, you can rebroadcast the same transaction with a higher fee. The network will prefer the new version and drop the old one. Available on Bitcoin and Ethereum. Check if your wallet (e.g. Electrum, MetaMask) supports it.
Option 2 — Child-pays-for-parent (CPFP) — Bitcoin
If RBF is not available, you can spend the unconfirmed output in a new transaction with a very high fee. The miner is incentivised to confirm both transactions together to claim the combined fee.
Option 3 — Wait it out
Most nodes drop transactions from their mempool after 2–14 days if they remain unconfirmed. Once dropped, the transaction effectively never happened and your funds remain in your wallet. Nothing is lost — just your time.
Important: A transaction stuck in the mempool has NOT left your wallet yet. Your funds are not lost — they are simply waiting. Do not panic and do not try to send the same funds again from a different address without cancelling the original transaction first, or you may attempt a double spend which will be rejected.
How to check the mempool and your transaction status
You can monitor mempool conditions in real time before sending a transaction, to choose the right fee:
₿ Bitcoin mempool tools
mempool.space — the most popular Bitcoin mempool explorer. Shows pending transactions, fee estimates, block predictions and historical mempool size.
Use it to decide what fee to set before sending Bitcoin.
🔷 Ethereum gas tools
etherscan.io/gastracker — shows current gas prices (slow/standard/fast) in Gwei.
ethgasstation.info — fee estimates for different confirmation speeds.
Most wallets (MetaMask, etc.) pull these estimates automatically.
To check the status of a specific transaction, use Abarai’s transaction checker or a blockchain explorer for the relevant network.
The mempool and MEV — the dark side of the waiting room
Because the mempool is public, sophisticated actors can see all pending transactions before they are confirmed. This creates a practice known as MEV (Maximal Extractable Value).
🤖 Front-running
A bot sees your large buy order for a token in the mempool. It places an identical buy order with a higher gas fee — getting confirmed first. The token price rises. Then it sells immediately, pocketing the difference. You pay more than you should have.
🥪 Sandwich attacks
A more aggressive form of front-running. A bot places a transaction before yours (driving up the price) and another after yours (selling at the inflated price). Your transaction is sandwiched — you get a worse execution price.
🛡️ How to protect yourself
Use slippage protection in DeFi apps. Set a low maximum slippage tolerance. Use MEV-protected RPC endpoints (e.g. Flashbots Protect on Ethereum). Avoid very large trades on low-liquidity tokens.
Frequently asked questions about the mempool
What is the mempool in crypto?
The mempool (memory pool) is a temporary holding area on each blockchain node where valid but unconfirmed transactions wait before being included in a block. Every transaction passes through the mempool before it is confirmed on-chain.
Why is my Bitcoin transaction stuck in the mempool?
Your transaction fee was likely too low. Miners prioritise higher-fee transactions. If the mempool is congested, low-fee transactions can wait hours or even days. Use Replace-by-Fee (RBF) to increase your fee, or wait for congestion to ease.
Can I lose funds if a transaction is stuck in the mempool?
No. If a transaction never confirms and is eventually dropped from the mempool (typically after 2–14 days), your funds remain in your wallet as if the transaction never happened. Nothing is lost.
How can I speed up a stuck transaction?
On Bitcoin: use Replace-by-Fee (RBF) if your wallet supports it, or Child-Pays-for-Parent (CPFP). On Ethereum: resubmit the same transaction with a higher gas fee using the same nonce. Most modern wallets (MetaMask, Electrum) have a built-in “speed up” button.
Does every blockchain have a mempool?
Most blockchain networks that use a block structure have some form of mempool. However, some faster networks like Solana have redesigned their transaction processing to minimise mempool delays — Solana’s Proof of History creates a verifiable order of events that reduces waiting time to milliseconds.
Why does the mempool affect gas fees on Ethereum?
When many transactions compete for limited block space, users increase their gas fee to get priority. The more congested the mempool, the higher fees must be to guarantee timely confirmation. This is why gas fees spike during popular NFT mints, token launches or major DeFi events.
What is MEV and how does it relate to the mempool?
MEV (Maximal Extractable Value) refers to profit extracted by miners or bots by exploiting the public visibility of the mempool. Since anyone can see pending transactions, bots can reorder or insert transactions to profit — through front-running, sandwich attacks or arbitrage.
mempool
what is mempool
transaction fees
bitcoin mempool
gas fees
unconfirmed transaction
MEV
crypto dictionary
what is mempool
transaction fees
bitcoin mempool
gas fees
unconfirmed transaction
MEV
crypto dictionary