The Mempool and Transaction Propagation

10 min readarticleIncludes quiz · 5 questions

The mempool (memory pool) is where unconfirmed transactions wait for miners to include them in blocks. Think of it as a waiting room. Each node has its own mempool, prioritizing transactions by fee. Understanding the mempool helps you set appropriate fees and predict confirmation times.

What is the Mempool?

  • •Definition: Each node's collection of valid, unconfirmed transactions waiting to be mined.
  • •Decentralized: Every node maintains its own mempool (they're similar but not identical).
  • •Fee market: Transactions compete for limited block space based on fees.
  • •Dynamic: Size fluctuates based on network activity.
  • •Eviction: Low-fee transactions can be dropped if mempool gets too full.
Mempool Visualization
Mempool Visualization

Key Definitions:

  • •Mempool: Memory pool of unconfirmed transactions at a node.
  • •Mempool size: Total vBytes of transactions waiting (measured in MB or vMB).
  • •Fee layers: Visualization of mempool organized by fee rate (sat/vB).
  • •Next block: Highest-fee transactions likely to be mined in the next block.
  • •Purging: Dropping low-fee transactions to make room (typically after 2 weeks).
  • •Mempool congestion: When mempool is large, indicating high demand for block space.

How the Mempool Works:

1. Transaction broadcast: Nodes receive new transactions from peers. 2. Validation: Each node validates the transaction independently. 3. Add to mempool: If valid, node stores transaction in memory. 4. Prioritization: Nodes sort mempool by fee rate (sat/vB), highest first. 5. Relay to miners: Mining nodes have access to these transactions. 6. Block template: Miners select highest-fee transactions to maximize profit. 7. Remove when confirmed: Once transaction is in a block, it's removed from mempool.

Mempool States:

Empty (< 10 MB)

  • •Low network activity
  • •Most transactions confirm quickly
  • •Even 1 sat/vB transactions confirm
  • •Common on weekends and during bear markets

Moderate (10-100 MB)

Congested (100-300 MB)

  • •High activity (bull markets, NFT mints, etc.)
  • •100+ sat/vB needed for fast confirmation
  • •Low-fee transactions can take hours or days

Extremely Congested (300+ MB)

  • •Network under heavy load
  • •500+ sat/vB for next block
  • •Transactions under 10 sat/vB might be purged
  • •Can last days to weeks
Mempool Congestion Levels
Mempool Congestion Levels

Why Mempools Differ Between Nodes:

Factors causing differences:

  • •Propagation timing: Transactions reach nodes at slightly different times
  • •Minimum fee policies: Some nodes filter out very-low-fee transactions
  • •Memory limits: Nodes have different mempool size limits
  • •Custom policies: Some nodes have custom rules about what transactions to accept
  • •Network partitions: Temporarily isolated nodes miss transactions

Typical differences:

  • •Usually 95-99% the same
  • •Differences are minor (seconds to minutes)
  • •All nodes see high-fee transactions quickly
  • •Low-fee transactions might not reach all nodes

Mempool Economics:

Transaction auction:

  • •Block space is scarce (~1 MB every 10 minutes)
  • •Transactions bid for space with fees
  • •Miners are rational profit-maximizers
  • •Higher fee = higher priority

Example:

  • •Transactions with 200+ sat/vB: Confirm immediately
  • •50-200 sat/vB: Wait 1-5 blocks (~10-50 minutes)
  • •10-50 sat/vB: Wait hours to a day
  • •<10 sat/vB: May never confirm or get purged

When fees spike:

  • •Major news (bullish events)
  • •NFT or token mints on Bitcoin (Ordinals, BRC-20)
  • •Exchange movements (whales moving funds)
  • •Arbitrage opportunities

Mempool Visualization Tools:

mempool.space (most popular)

  • •Real-time mempool size and fee distribution
  • •Shows which transactions will confirm in next blocks
  • •Historical data and charts
  • •Per-block fee breakdown

Features:

Other tools:

Mempool Fee Layers
Mempool Fee Layers

Transaction Lifecycle in Mempool:

Phase 1: Just broadcast (0-10 seconds)

  • •Propagating across network
  • •Being validated by nodes
  • •Added to mempools

Phase 2: Waiting (minutes to hours)

  • •Sitting in mempool
  • •Competing with other transactions
  • •Position depends on fee rate

Phase 3: Mined (when a block includes it)

Phase 4: Confirmed (6+ blocks later)

  • •Deeply buried in blockchain
  • •Economically irreversible
  • •Considered final

Mempool Purging:

Why transactions get dropped:

  • •Time: Default is 14 days (2 weeks) in mempool without confirmation
  • •Space: Mempool size limit reached (typically 300-450 MB)
  • •Fee: Transaction fee too low to be competitive

What happens:

  • •Node removes transaction from its mempool
  • •Funds "return" to sender (UTXO never actually spent)
  • •Sender can try again with higher fee

Preventing purging:

  • •Set adequate fee initially
  • •Use RBF to bump fee if stuck
  • •Some services rebroadcast periodically

Test Your Knowledge

This lesson includes a 5-question quiz (passing score: 75%).

Quiz functionality available in the mobile app.