The Mempool and Transaction Propagation

• •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: 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.

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)

• •Normal activity
• •10-50 sat/vB for next-block confirmation
• •Lower fees wait a few blocks

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

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

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.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:

• •Fee estimates (slow, medium, fast)
• •Transaction tracking
• •Block explorer
• •Lightning Network explorer

Other tools:

• •bitcoinfees.net: Simple fee recommendations
• •johoe.de/mempool: Classic mempool chart
• •Your node (bitcoin-cli getmempoolinfo)

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)

• •Removed from mempool
• •Now has 1 confirmation
• •Considered "pending" (not final yet)

Phase 4: Confirmed (6+ blocks later)

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

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