Overview
Dust refers to tiny amounts of bitcoin held in transaction outputs that are so small they cost more in fees to spend than they are worth. These economically unspendable outputs bloat the UTXO set that every full node must maintain, consuming memory and storage resources across the network. To mitigate this, Bitcoin Core enforces a "dust limit" -- a minimum output value below which transactions are rejected from the mempool by default.
How the Dust Limit Is Calculated
The dust limit is not a fixed amount; it depends on the output script type and the current dust relay fee rate (default: 3 sat/vB):
Dust limit = cost to create the output + cost to spend it
= (output_size + input_size) * dust_relay_fee_rate
Approximate dust limits (at 3 sat/vB):
┌─────────────────────┬──────────────┐
│ Output Type │ Dust Limit │
├─────────────────────┼──────────────┤
│ P2PKH │ 546 sats │
│ P2SH │ 540 sats │
│ P2WPKH (SegWit) │ 294 sats │
│ P2WSH │ 330 sats │
│ P2TR (Taproot) │ 330 sats │
└─────────────────────┴──────────────┘
Why Dust Is a Problem
- UTXO set bloat: Every unspent output must be tracked by all full nodes in RAM for fast validation. Millions of dust UTXOs waste resources.
- Economic loss: Dust represents bitcoin that effectively cannot be moved, reducing the circulating supply in a practical sense.
- Privacy attacks: "Dust attacks" involve sending tiny amounts to many addresses to track spending patterns through chain analysis when the dust is eventually consolidated.
Dust Attacks
A dust attack is a de-anonymization technique:
- An attacker sends tiny amounts (just above the dust limit) to many addresses
- When recipients unknowingly include the dust in a future transaction, it links their addresses together
- The attacker uses this information for blockchain surveillance
Mitigation Strategies
- Coin control: Many wallets allow users to manually select which UTXOs to spend, enabling users to avoid spending dust inputs or quarantine suspicious dust
- Consolidation during low fees: Users can merge small UTXOs when fees are low
- Labeling: Mark incoming dust from unknown sources and avoid spending it alongside other UTXOs
- SegWit/Taproot: Using more efficient output types lowers the dust threshold, making more small outputs economically spendable