What is the Bitcoin Mempool?

Introduction

Have you ever wondered why sometimes your Bitcoin transaction confirms in 10 minutes, but other times it takes hours or even days? Or why fees vary so much - sometimes $2, sometimes $50 or more? The answer lies in the mempool.

The mempool is one of the most important but least understood parts of the Bitcoin ecosystem. It's the "waiting room" where all transactions wait to be included in a block. Understanding the mempool helps you understand why fees vary, how to prioritize your transactions, and when it's better to wait or pay more.

This guide will explain what the mempool is, how it works, and how it affects your transactions. You'll understand pending transactions, priority, fees, and congestion. Our goal is to explain why fees vary so much, using real cases to illustrate.

By the end of this guide, you'll be able to understand the current state of the mempool and make smarter decisions about when to send transactions and how much to pay in fees.

What is the Mempool?

Definition

Mempool (Memory Pool) is the set of all valid Bitcoin transactions that have been broadcast to the network but have not yet been included in a confirmed block.

In simple terms: the mempool is like a "queue" of transactions waiting to be processed. When you send a Bitcoin transaction, it first goes to the mempool. Miners then choose which transactions from the mempool to include in the next block they're mining.

Simple Analogy

Think of the mempool as a very busy restaurant:

Waiting line (mempool): People waiting for a table
Waiters (miners): Choose who to serve first
Tables (blocks): Limited space, only fits certain number of people
High tip (high fee): Those who pay more tip are served first
Low tip (low fee): May wait a long time

In practice:

Mempool = transaction queue
Miners = choose which to include
Blocks = limited space (~2,500-3,500 transactions)
Fee = inclusion priority

Mempool Characteristics

1. Not centralized:

Each Bitcoin node maintains its own mempool
Nodes can have different mempools
Some nodes have larger mempools, others smaller

2. Dynamic:

Transactions enter constantly
Transactions exit when mined
Size varies over time

3. Volatile:

During congestion, mempool can have thousands of transactions
In quiet moments, may be almost empty
Can change drastically in hours

4. Competitive:

Transactions compete for space in next block
Those paying more fee have priority
Low fee transactions may wait long

How Does the Mempool Work?

Transaction Lifecycle

1. Creation:

You create transaction in your wallet
Define inputs, outputs, and fee

2. Broadcast:

Wallet sends transaction to nearby Bitcoin nodes
Transaction is propagated through network

3. Validation:

Nodes verify if transaction is valid
Verify signatures, balances, format

4. Entry into Mempool:

If valid, transaction enters mempool
Waits to be mined

5. Selection by Miner:

Miner chooses transactions from mempool
Usually prioritizes by fee (sat/vB)

6. Inclusion in Block:

Transaction is included in block
Removed from mempool
Block is mined and added to blockchain

7. Confirmation:

Transaction receives first confirmation
No longer pending

What Happens in the Mempool?

In the mempool, transactions are organized mainly by fee:

Typical structure:

Mempool (ordered by priority):
├── Very high fee (50+ sat/vB): Confirm in ~10 minutes
├── High fee (20-50 sat/vB): Confirm in 10-30 minutes
├── Medium fee (10-20 sat/vB): Confirm in 30-120 minutes
├── Low fee (5-10 sat/vB): Confirm in 2-24 hours
└── Very low fee (less than 5 sat/vB): May wait days or never confirm

Miners choose:

Transactions with higher fee per byte (sat/vB)
Fill block until limit (~4 MB or weight limit)
Leave low fee transactions for later

Pending Transactions

What Are Pending Transactions?

Pending transactions are transactions in the mempool, waiting for confirmation in a block.

Characteristics:

Are on network (already broadcast)
Not yet confirmed
Can be confirmed in next block
Or may wait long time

Why Do They Stay Pending?

Common reasons:

1. Fee too low:

Fee lower than other transactions
Miners prioritize others
May wait long time

2. Network congestion:

Many transactions competing
Little space available in blocks
Transactions need to wait

3. Transaction size:

Large transactions occupy more space
Miners may prefer many small transactions
Pay less per byte

4. Miner priority:

Some miners have preferences
May prioritize certain types
But usually prioritize by fee

How to See Pending Transactions?

Blockchain explorers show mempool:

Sites like Blockchain.com, Blockstream.info
Show pending transactions
Show estimated confirmation fee
Show mempool size

What you see:

List of transactions waiting
Fee of each transaction
Expected confirmation time
Mempool statistics

Mempool Priority

How Does Priority Work?

Priority is determined mainly by fee per byte (sat/vB):

Simple formula:

Priority = Total Fee / Transaction Size (in bytes)

Example:

Transaction A: Fee 0.001 BTC, Size 250 bytes
Priority = 0.001 BTC / 250 bytes = 4,000 sat/vB (very high)
Transaction B: Fee 0.0001 BTC, Size 500 bytes
Priority = 0.0001 BTC / 500 bytes = 200 sat/vB (medium)

Transaction A has priority over B (higher fee per byte).

Factors Affecting Priority

1. Fee per Byte (sat/vB):

Most important factor
Higher = higher priority
Miners want to maximize revenue

2. Transaction Size:

Smaller transactions may be preferred
More transactions fit in block
But total fee also matters

3. Transaction Age:

Some miners consider time in mempool
Very old transactions may have slightly higher priority
But fee is still more important

4. Replace-by-Fee (RBF):

Allows increasing fee of pending transaction
New transaction replaces old one (if accepts RBF)
Useful if fee became too low

Priority Example

Scenario: Mempool has 5,000 transactions, block fits ~3,000

Transactions in mempool (ordered by priority):

1,000 transactions with 50+ sat/vB (high priority)
1,500 transactions with 20-50 sat/vB (medium-high priority)
1,000 transactions with 10-20 sat/vB (medium priority)
1,500 transactions with 5-10 sat/vB (low priority)

Next block:

Miner includes all 1,000 high priority
Miner includes all 1,500 medium-high
Miner includes 500 of the 1,000 medium priority
Block is full

Result:

3,000 transactions confirmed
2,000 transactions still pending
Low priority transactions continue waiting

Fees and the Mempool

Why Do Fees Vary So Much?

Fees vary based on competition for space in the mempool:

When mempool is empty:

Few transactions competing
Low fee is enough (1-5 sat/vB)
Fast confirmation even with low fee

When mempool is full:

Many transactions competing
Need to pay more to be prioritized
High fee necessary (20-100+ sat/vB)

Relationship Between Mempool and Fees

Basic law of supply and demand:

Supply (available space):

Each block has limited space (~3,000 transactions)
One block every ~10 minutes
Capacity: ~7 transactions per second

Demand (transactions in mempool):

Can vary from hundreds to tens of thousands
Increases during important events
Decreases in quiet moments

Equilibrium:

High demand + low supply = high fees
Low demand + normal supply = low fees

Real Case: December 2017

Situation:

Mempool had more than 200,000 pending transactions
Blocks were full
Average fees reached more than $30

What happened:

Many people wanting to use Bitcoin
Network couldn't process everything quickly
Transactions with low fees got stuck for days
Many people needed to increase fees

Result:

Very high fees for several weeks
Slow transactions
Demonstrated need for scalability solutions

Real Case: Quiet Periods

Situation (example: June 2023):

Mempool relatively empty
Few transactions competing
Very low fees

What happened:

Fees of 1-3 sat/vB confirmed in 10-30 minutes
Many transactions with low fee were included quickly
Network worked well

Result:

Affordable fees
Fast confirmations
Better experience for users

Mempool Congestion

What is Congestion?

Congestion happens when there are more transactions in the mempool than can be processed in the next blocks.

Signs of congestion:

Mempool with many transactions (10,000+)
Fees increasing
Confirmation time increasing
Low fee transactions getting stuck

Causes of Congestion

1. Important Events:

Bitcoin halving
Large exchange movements
Important news
Adoption spikes

2. Transaction Spam:

Someone sends many low fee transactions
Occupies space in mempool
Makes it difficult for legitimate transactions

3. Block Limits:

Maximum block size (~4 MB or weight limit)
Time between blocks (~10 minutes)
Naturally limited capacity

4. High Transaction Volume:

Many people using Bitcoin at same time
Demand exceeds capacity
Natural congestion

Impact of Congestion

For Users:

Option 1: Wait:

Low fee transactions get stuck
May wait hours or days
May be acceptable if not in a hurry

Option 2: Pay More:

Increase fee for priority
Use Replace-by-Fee (RBF)
Faster confirmation, but costs more

For the Network:

Fees increase
Slow transactions
Bad experience for some users
May affect adoption

Real Case: May 2021 Congestion

Situation:

Mempool reached more than 150,000 transactions
Average fees of $10-20
Many transactions stuck

Causes:

Great interest in Bitcoin
Many NFT and token transactions
Overloaded network

Impact:

Very slow transactions
Very high fees
Some people stopped using
Many migrated to Lightning Network

Solution:

People increased fees
Some waited days
Others used Lightning Network
Congestion decreased gradually

How the Mempool Affects You

Choosing Appropriate Fee

To choose appropriate fee, check the mempool:

1. See Minimum Recommended Fee:

Blockchain explorers show this
Mempool.space shows recommendations
Wallets usually suggest fees

2. Consider Urgency:

If need fast: pay more
If can wait: pay less
Check estimated time

3. Monitor Mempool:

See how many transactions are pending
See average fee being accepted
Adjust your transaction as needed

Strategies for Different Situations

1. Urgent Transaction:

Check fee recommended for ~10 minutes
Pay a bit above to guarantee
Use high fee (30-50+ sat/vB during congestion)

2. Normal Transaction:

Use fee recommended for 1-3 hours
Usually 10-20 sat/vB
Good cost-benefit ratio

3. Non-Urgent Transaction:

Use low fee (1-5 sat/vB)
May wait hours or days
Saves money

4. During Congestion:

Avoid if possible
If needed, pay high fee
Or wait for congestion to pass

Tools to Monitor Mempool

1. Mempool.space:

Excellent mempool visualization
Fee graphs
Time estimates
Detailed statistics

2. Blockchain Explorers:

Blockchain.com
Blockstream.info
Show pending transactions
Estimate fees

3. Wallets:

Many wallets show estimates
Suggest fees automatically
Based on mempool state

Real Cases of Fee Variation

Case 1: Normal Day

Situation:

Mempool: ~5,000 transactions
Average fee: 15 sat/vB
Confirmation time: 30-60 minutes

$100 transaction:

Recommended fee: 15 sat/vB
Fee paid: ~$2
Confirmed in: 45 minutes

Analysis: Worked well, reasonable fee.

Case 2: Moderate Congestion

Situation:

Mempool: ~25,000 transactions
Average fee: 50 sat/vB
Confirmation time: 2-4 hours

$100 transaction:

Recommended fee: 50 sat/vB
Fee paid: ~$8
Confirmed in: 3 hours

Analysis: Higher fee, but still acceptable.

Case 3: Severe Congestion

Situation:

Mempool: ~100,000+ transactions
Average fee: 150 sat/vB
Confirmation time: 6-24 hours

$100 transaction:

Option 1: Low fee (5 sat/vB) = $0.50
- Result: Got stuck for 3 days
Option 2: High fee (150 sat/vB) = $30
- Result: Confirmed in 8 hours

Analysis: Need to choose between waiting long or paying much.

Case 4: Very Quiet Period

Situation:

Mempool: ~500 transactions
Average fee: 2 sat/vB
Confirmation time: 10-20 minutes

$100 transaction:

Recommended fee: 2 sat/vB
Fee paid: ~$0.20
Confirmed in: 15 minutes

Analysis: Very low fee, fast confirmation, ideal!

Optimizing Transactions in Mempool

Tips to Reduce Fees

1. Choose Less Busy Times:

Weekends may be quieter
Hours of less activity
Mempool may be smaller

2. Use SegWit:

Addresses starting with "bc1"
Smaller transactions
Fewer bytes = less total fee for same sat/vB

3. Combine UTXOs Before:

Fewer inputs = smaller transaction
If have many small UTXOs, combine first
Reduces transaction size

4. Avoid Congestion:

Monitor mempool before sending
If very full, wait if possible
Or pay appropriate fee

Replace-by-Fee (RBF)

What RBF is:

Allows increasing fee of pending transaction
New transaction replaces old one (with higher fee)
Useful if fee became too low

How to use:

Transaction got stuck with low fee
Create new transaction with same structure
Increase fee significantly
Send new transaction (replaces old one)

When to use:

Transaction stuck in mempool
Need fast confirmation
Original fee was too low

Frequently Asked Questions

Can my transaction get stuck forever?

Technically not forever, but can take very long. Transactions can stay in mempool for days or weeks if fee is too low. May eventually be removed from mempool if they get very old.

Can I cancel a pending transaction?

Not directly, but can use Replace-by-Fee (RBF) if transaction supports it. Or send to yourself with higher fee to "push" old transaction.

Why do mempools of different sites show different numbers?

Each node maintains its own mempool. Some nodes have larger mempools, others smaller. Depends on which transactions each node received and validated. But all converge to similar values.

Minimum fee to always confirm?

There's no absolute guarantee. But generally 1-2 sat/vB eventually confirm, may take long. To guarantee confirmation in reasonable time, pay fee recommended by mempool.

Will mempool improve in the future?

Yes. Solutions like Lightning Network reduce load on mempool. Taproot also helps. But mempool will always exist as long as there's block size limit.

Conclusion

The mempool is fundamental to understanding how Bitcoin works in practice. It's where all transactions compete for space in blocks, and where you can see in real time the "health" of the Bitcoin network.

The main points you need to understand are:

Mempool is the queue of pending transactions - waiting for confirmation
Priority is determined by fee - those paying more go first
Fees vary with congestion - more transactions = higher fees
You can optimize - choose times, use SegWit, monitor mempool

Understanding the mempool gives you power to:

Choose smart fees
Know when to wait or pay more
Understand why transactions are slow
Optimize your Bitcoin payments

Fees vary because Bitcoin has limited capacity (blocks of limited size), and when many people want to use it at the same time, it creates competition. It's basic economics: high demand and limited supply = higher prices (fees).

As you use Bitcoin more, you'll learn to "read" the mempool and make better decisions about your transactions. It's a valuable skill that can save money and time. The mempool isn't perfect, but it's transparent - you can always see what's happening and adjust your strategies.