What is the Mempool?
The mempool (memory pool) is the waiting room for unconfirmed Bitcoin transactions. When you broadcast a transaction, it sits in the mempool until a miner includes it in a block.
Key Concept: Block Space Scarcity
- New blocks are created approximately every 10 minutes
- Each block has limited space (~4 MB effective capacity)
- When demand exceeds supply, a fee market emerges
- Higher fees = Higher priority in the queue
Live Mempool Visualization
👆 Click "Add Transaction" to see how transactions enter the mempool
How Do Miners Prioritize Transactions?
Miners are economically incentivized to maximize revenue. They select transactions that pay the highest fee per byte (sat/vB).
Miner Incentives
- Block Reward: Currently 3.125 BTC per block (halves every ~4 years)
- Transaction Fees: Sum of all fees from included transactions
- Total Revenue: Block Reward + Transaction Fees
- Strategy: Fill block with highest-paying transactions first
Mempool (Sorted by Fee Rate)
Next Block
👆 Click "Mine Block" to see which transactions get confirmed first
Understanding the Fee Market
Bitcoin's fee market is pure supply and demand. Block space is the scarce resource, and transaction fees are the price.
Fee Market Principles
- Supply: ~4 MB of block space every 10 minutes (fixed)
- Demand: Variable number of transactions wanting confirmation
- Price Discovery: Users compete by offering higher fees
- Market Clearing: Fees adjust until supply = demand
Network Scenarios
Current Mempool Fee Distribution
Your Transaction
Your transaction is in the top 50% of the mempool
Practice: Interactive Fee & Transaction Exercises
Test your understanding with hands-on exercises: fee estimation, transaction selection, and batch analysis.
🎯 Exercise 1: Transaction Selection
You're a miner building a block. Select transactions to maximize your revenue. Click transactions to add them to your block.
Available Transactions (Mempool)
Your Block
Click transactions to add them to your block
📦 Exercise 2: Batch Transaction Analysis
Group these transactions by characteristics. Drag transactions into the correct categories.
High Priority (50+ sat/vB)
RBF Enabled
UTXO Consolidation (Many Inputs)
📊 Exercise 3: Fee Estimation Scenarios
Choose the right fee for different situations.
Scenario 1: Paying for Coffee ☕
You're buying coffee with Bitcoin (via Lightning). The Lightning channel needs to be opened with an on-chain transaction. You're not in a rush.
Current Market Conditions: Medium demand
- Next block: 20 sat/vB
- Within 6 blocks: 8 sat/vB
- Within 24 hours: 2 sat/vB
Scenario 2: Emergency Exchange Withdrawal 🚨
You need to withdraw Bitcoin from an exchange urgently to take advantage of a time-sensitive trading opportunity. Speed matters.
Current Market Conditions: High demand (congestion)
- Next block: 80 sat/vB
- Within 6 blocks: 40 sat/vB
- Within 24 hours: 20 sat/vB
Scenario 3: Moving to Cold Storage 🔐
You're consolidating UTXOs and moving funds to cold storage. There's no time pressure.
Current Market Conditions: Low demand (mempool clearing)
- Next block: 5 sat/vB
- Within 6 blocks: 2 sat/vB
- Within 24 hours: 1 sat/vB
Key Takeaways: The Fee Market
Mempool = Waiting Room
Unconfirmed transactions wait in the mempool until a miner includes them in a block.
Miners Optimize Revenue
Miners select transactions with the highest fee per byte (sat/vB) to maximize earnings.
Supply & Demand
Block space is scarce. When demand is high, fees rise. When demand is low, fees fall.
Trade-off: Speed vs. Cost
Higher fees = Faster confirmation. Lower fees = Longer wait. Choose based on urgency.
Fee Estimation is Critical
Always check current mempool conditions before broadcasting. Use fee estimation tools.
Advanced: RBF & CPFP
If your transaction gets stuck, you can use Replace-By-Fee (RBF) or Child-Pays-For-Parent (CPFP) to accelerate it.
Fee Calculation Formula
Total Fee = Transaction Size (bytes) × Fee Rate (sat/vB)
Example: 250 bytes × 10 sat/vB = 2,500 sats
Best Practices
- ✅ Always enable RBF (Replace-By-Fee) for flexibility
- ✅ Check mempool conditions before broadcasting transactions
- ✅ Use fee estimation tools (mempool.space, your wallet's estimator)
- ✅ For non-urgent transactions, wait for low-demand periods (weekends)
- ✅ Consolidate UTXOs during low-fee periods to save on future fees
- ❌ Don't overpay for non-urgent transactions
- ❌ Don't use 1 sat/vB during high demand unless you can wait days
🔴 Live Mempool Data
Now that you understand the concepts, explore the real Bitcoin mempool:
🔗 Explore on mempool.space
🧠 Live Mempool Analysis Challenge
Based on the current mempool conditions shown above, answer these questions:
Question 1: You need to send a 250-byte transaction for a payment that should confirm within 1 hour. What fee rate should you use?
Question 2: Looking at the current mempool size, how would you describe network congestion?
Question 3: Calculate: If you send a 400-byte transaction at the Medium Priority rate, what will your total fee be?
Test Your Knowledge
Ready to apply what you've learned?