Mempool
The mempool is the waiting area of transactions that have been broadcast but not yet included in a block. Transactions offering higher fees are usually picked first.
The mempool is the waiting room for transactions that have been sent but not yet put in a block. How crowded it is drives fees and waiting times, and because it is often public, others can see your trade before it lands.
- Category
- Cryptocurrency and Blockchain
- Reading time
- 3 min
- Also called
- Memory pool, Pending transactions, Transaction pool
- Updated
The mempool in plain words
The mempool is each node's local waiting room for transactions that haven't made it into a block yet.1
There is no single shared mempool; nodes pass transactions to peers, so each copy differs slightly.1
Pending transactions are visible to everyone, which lets others react before they are confirmed.2
How a transaction moves through the mempool
Sign and send
Your wallet sends the signed transaction to a node.
Check and store
The node adds it to its local mempool and broadcasts it to other nodes.3
Meet the fee floor
A transaction must pay a minimum relay fee to be accepted into the memory pool.4
Wait for selection
Block producers pick from the pool, usually favouring higher fees.
Watch for front-runners
Anyone monitoring the mempool can try to front run it with a higher gas offer.5
Leave the pool
Once included in a block, the transaction is removed from the mempool.
Worked examples
Example 1 — Crowded pool
About 10 blocksHypothetically 30,000 transactions wait and blocks take about 3,000 each.
| Blocks to clear | 30,000 ÷ 3,000 = 10 blocks |
| At 12 seconds a block | 10 × 12 s = 120 s |
Each node holds these pending transactions until they are included in a block.1
Example 2 — Being front-run
−$100A hypothetical trader sends a large buy; a bot copies it with a higher tip and pushes the price 1% higher before the trader's fill on $10,000.
| Extra cost | $10,000 × 1% = $100 |
This is possible because participants monitoring the mempool can front run unconfirmed transactions with a higher amount of gas.5
Example 3 — Private route
$100 savedThe same trader sends through a private pool visible only to selected validators.
| Public visibility | None before inclusion |
| Front-run cost | $0 in this scenario |
Private mempools make transactions visible to specific validators only.2
Public vs private mempool
| Aspect | Public mempool | Private mempool |
|---|---|---|
| Who can see | Any node | Selected builders or validators |
| Front-run risk | Higher | Lower |
| Reach | Whole network | Limited set of block producers |
| Trust needed | Low | Trust the private operator |
How traders use mempool data
Fee setting
Judge the tip needed versus Trading Fees and Commissions.
Order protection
Reduce Slippage from front-running.
Flow reading
Large pending swaps hint at Order Flow.
Timing
Avoid sending during Volatility spikes.
Advantages and disadvantages
Advantages
- Shows network demand in real time.
- Helps set fees sensibly.
- Public pools keep inclusion open to all.
Disadvantages
- Public visibility invites front-running.
- Each node's view differs.
- Low-fee transactions can wait a long time.
- Private routes add trust in an operator.
Going deeper
Read as far as you need. Each level builds on the one before it.
A waiting room
Beginner
Pending transactions sit in each node's local mempool until a block includes them.1
Many mempools
Intermediate
There is no single shared mempool; the node broadcasts transactions to peers who add them to their own copy.3
Visibility and extraction
Advanced
Public visibility lets agents analyse transactions before inclusion; private mempools make them visible to specific validators only.2
Common mistakes
Setting a tiny fee
The transaction may sit for hours.
Broadcasting large swaps publicly
Invites sandwich attacks.
Assuming one global queue
Nodes hold different pools.
Resending without replacing
Duplicates can confuse wallets.
Check your understanding
Test yourself before moving on. Each answer comes with an explanation.
Key points to remember
- The mempool holds pending transactions.
- Each node has its own copy.
- Fees decide who gets in first.
- Public visibility enables front-running.
- Private pools trade visibility for trust.
Frequently asked questions
Why is my transaction stuck?
Can I see the mempool?
Does Bitcoin have a mempool too?
Further learning
- Order Flow
Reading pending demand.
- Slippage
Cost of being front-run.
- Trading Fees and Commissions
Setting fees sensibly.
- Market Order
Why market orders are exposed.
Sources
This article is written in our own words. The sources below support the specific facts, rules and figures marked with a number in the text. Links open the original publisher’s page.
- ethereum.org. Ethereum Glossary (opens in a new tab) Checked .
- European Securities and Markets Authority (ESMA). Maximal Extractable Value: Implications for crypto markets (2025) (opens in a new tab) Checked .
- ethereum.org. Proof-of-stake (PoS) (opens in a new tab) Checked .
- Bitcoin Project (developer.bitcoin.org). Glossary (opens in a new tab) Checked .
- European Securities and Markets Authority (ESMA). Decentralised Finance in the EU: Developments and risks (TRV, Oct 2023) (opens in a new tab) Checked .
Educational content, not financial advice. Examples use hypothetical prices and simplified costs; actual fees, margin rules and regulations vary by broker, market and country. We are not affiliated with any publisher linked above. Last reviewed . Report an error or request a source credit.