Understanding Sub-1 sat/vB Bitcoin Transactions
What is "sat/vB" and why is it important?
In the Bitcoin network, every transaction takes up space in a block, measured in virtual bytes (vB), and transaction fees are paid using satoshis (sat), therefore the network fee rate is satoshi per virtual byte (sat/vB).
Node operators would prioritize confirming transactions offering higher fees (more sat per vB).
Higher fee rate → Faster confirmation
Lower fee rate → Delayed confirmation, or potentially never confirmed
Why Do Most Bitcoin Nodes Require fees above 1 sat/vB?
Bitcoin Core enforces a default minimum relay fee policy, which only accepts and relays transactions with ≥ 1 sat/vB. Most nodes follow this rule to:
Ensure miner rewards
Discourage transaction spam
Maintain efficient mempool operation
This 1 sat/vB threshold has become a network-wide consensus rule. However, it is is more of a policy and not a protocol constraint.
How Sub 1 sat/vB Transactions Can Be Processed
Given the default minimum relay fee policy of 1 sat/vB, Sub-1 sat/vB transaction can only be processed through:
Direct submission to miners, or
Private relay services (e.g., Slipstream).
This means that confirmation of these transactions is not guaranteed, and only occurs during when network is less congested AND if miners accept sub-1 sat policies.
✅ When Sub-1 sat/vB May Be Appropriate:
When the mempool is completely empty and competition is low
For non-urgent, time-insensitive transactions
For experiments or testing purposes
❌ When NOT to Use Sub-1 sat/vB:
Urgent transfers or large fund movements
Time-sensitive operations like snapshots, mints, or on-chain events
When generating a large number of small UTXOs, which could lead to future fee traps
🔧 Best Practices:
Only enable Sub-1 mode if you fully understand the risks
Always check the current mempool conditions before using low fees
Avoid merging multiple sub-1 UTXOs to prevent long-term fund lockups
Remember: if your sub-1 transaction gets stuck, accelerating it later can cost 20–200x more than the original fee
UniSat Supports Sub-1 sat/vB Transactions
UniSat provides support for sub-1 fee transactions across several tools:
UniSat Inscribe
When minting Ordinals or Runes, users can enable Sub-1 sat/vB Mode.
UniSat UTXO Management Tool on Bitcoin
Perform UTXO management in Sub-1 sat/vB Mode:
Reclaim BTC trapped in useless inscription UTXOs
Batch send, one-click merge, split, lock, and unlock Odinals, Runes, Alkanes UTXOs
Check which UTXOs are currently locked in sub-1 sat/vB transactions
Stay tuned for integration across more UniSat products.
Learn More
To dive deeper into Sub-1 sat/vB transactions and risks, refer to our full guide: https://docs.unisat.io/technical/understanding-sub-1-sat-vb-bitcoin-transactions/what-happens-when-bitcoin-fee-rates-drop-below-1-sat-vb
Disclaimer:
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