What Happens Below 1 sat/vB + The Hidden Risks of Sub-1 sat/vB Bitcoin Transactions
What Happens When Bitcoin Fee Rates Drop Below 1 sat/vB?
Bitcoin transaction fees are measured in satoshis per virtual byte (sat/vB). This unit helps miners prioritize transactions when adding blocks to the blockchain. Bitcoin's default relay policy (followed by most nodes) requires a minimum fee rate of 1 sat/vB for a transaction to be relayed across the network. This policy exists to:
Provide a minimum economic incentive for miners to include transactions.
Prevent spam by making it costly to flood the network.
Maintain efficient network operation by limiting mempool bloat.
Transactions paying less than 1 sat/vB (sub-1 sat/vB) are considered non-standard under this default policy.
However, as the ecosystem evolves there may be some demand for sub-1 sat/vB transactions and more occasions where the miner may accept these transactions such as when the mempool cleared recently.So how do these sub-1 sat/vB transactions work?
⏳ When Can Sub-1 sat/vB Transactions Be Confirmed?
Not Relayed: They are not broadcast or relayed by nodes enforcing the default policy. They will not enter the public mempool.
Confirmation only possible if:
The transaction is submitted directly to a miner (bypassing the public peer-to-peer network), or
The transaction is submitted via a private relay service (like Slipstream used by some pools) that accepts sub-1 sat/vB rates, or
Not Guaranteed: Even if directly submitted to a miner or pool, inclusion is never guaranteed.
Miners prioritize transactions based on their own fee thresholds and strategies. Sub-1 sat/vB transactions are the lowest priority and will only be mined when there is plenty of unused block space and the individual miner or mining pool explicitly chooses to accept and mine sub-1 sat/vB transactions using custom mempool policies.
💡 Potential Benefits of Sub-1 sat/vB Transactions
Lower Costs: If confirmed, they enable cheaper transactions, potentially benefiting low-value transfers or microtransactions. (It wouldn’t make sense if you want send $1 worth of BTC but have to spend $5 on gas right)
Reduced Minting Costs: Protocols relying heavily on on-chain transactions for operations like minting (e.g., Runes, Ordinals, brc-20 tickers) could see significant cost reductions.
The Hidden Risks of Sub-1 sat/vB Transactions
#1 Stuck Transactions:
Most mining pools currently do not mine transactions with 0.1 sat/vB or similar ultra-low fee rates
Your transaction may remain unconfirmed for days, weeks, or indefinitely
Funds become inaccessible - you can't spend the same UTXOs until the transaction confirms
Particularly problematic for time-sensitive operations such as mint or trade
#2 Dust UTXO Attacks
How the Attack Works:
Malicious actors send low-fee-rate UTXOs to your address
Later, your wallet automatically includes this UTXO when constructing a new transaction
The entire transaction inherits the low fee rate, making it unconfirmable
You must pay expensive RBF (Replace-By-Fee) acceleration costs to fix the issue
Why it’s sneaky:
Our wallet blocks UTXOs <600 sats, but attacks over 600 sats bypass this.
Merging these UTXOs (via wallet tools) spreads the poison to new transactions.
#3: Transaction Chain Limitations
The Technical Problem:
Bitcoin's mempool has a 25-transaction ancestor limit
Repeatedly spending 0.1 sat/vB transactions quickly approaches this limit
Once reached, new transactions cannot be broadcast
Regular users have no way to resolve this without technical tools
Resolution Challenges:
Only solution is using RBF (Replace-By-Fee) acceleration tools to replace earlier transactions
Current tool limitations - many RBF implementations have bugs and don't handle data cleanup properly
#4: Increased Spam Transactions:
Lowering the effective cost barrier significantly makes it cheaper for attackers to flood the network with spam or dust transactions (very small outputs), potentially disrupting normal operation.
🎯 Recommendations for Users
Avoid sub-1 sat/vB transactions unless you fully understand the risks
Be Aware of the limited ecosystem support for handling sub-1 sat/vB transactions properly and potential inconsistent behavior across different wallet implementations
Keep RBF tools available for emergency acceleration
We recommend checking the current Bitcoin network congestion. A fee rate of 1 sat/vB or lower means it may result in longer confirmation times. You may use mempool.space to view the fee rate and estimated confirmation time in real time.
Disclaimer:
The content of this document is for informational purposes only. It does not constitute (i) investment advice or recommendations, (ii) an offer, solicitation, or inducement to purchase, sell, or hold digital assets, or (iii) financial, accounting, legal, or tax advice. Digital assets (including FTs and NFTs) are subject to market volatility, involve high risk, regulatory changes, and may depreciate in value. The information is provided "as is," and we make no warranties regarding its accuracy. Please consult your legal, tax, or investment professionals to determine whether trading or holding digital assets is suitable for you.
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