Sats per vByte
Sats per vByte (sat/vB) is the standard unit for measuring Bitcoin transaction fee rates. It represents the number of satoshis paid per virtual byte of transaction data, determining how quickly miners will prioritize and include a transaction in a block.
How It Works
Virtual bytes (vbytes) are the unit of measurement introduced by SegWit to account for the discount applied to witness data. A transaction's weight in weight units is divided by four to get its size in virtual bytes. SegWit transactions benefit from this discount because their witness data (signatures) counts at one-quarter the weight of other transaction data, resulting in lower effective fee rates compared to legacy transactions.
Fee rates determine transaction priority. When the mempool is congested, miners select transactions with the highest sats/vByte rates first. A transaction paying 50 sat/vB will generally confirm before one paying 10 sat/vB. During low-demand periods, 1 sat/vB may be sufficient. During high demand, rates can spike to hundreds of sats/vByte. Monitoring the mempool before transacting is essential for setting appropriate fees.
Most wallet software provides fee estimation, but these estimates vary in accuracy. Some wallets overpay consistently, which adds up over time. Experienced self-custody users check mempool conditions using tools like their own node's mempool data or trusted fee estimators, then manually set fee rates appropriate for their urgency. Features like replace-by-fee (RBF) provide a safety net — you can start with a lower fee and bump it if needed.
Key Points
- Standard unit for Bitcoin transaction fee rates (satoshis per virtual byte)
- Virtual bytes account for SegWit's witness data discount
- Higher sat/vB rates mean faster confirmation during congestion
- Always check mempool conditions before setting fees
- Use RBF-enabled transactions to start low and bump fees if needed