Sats per vByte

Sats per vByte (sat/vB) is the standard way to express a Bitcoin transaction's fee rate: the number of satoshis paid per virtual byte of transaction data. Miners pack the most valuable transactions into limited block space, so fee rate — not the total fee — is what determines confirmation priority. A small transaction paying a high rate can outrank a large transaction paying a bigger total fee, because it buys more reward per unit of the scarce resource, which is space in the next block.

What a virtual byte is

Since the SegWit upgrade, block capacity is measured in weight units, capped at 4,000,000 per block. Most transaction data counts as 4 weight units per byte, while witness data — signatures and other unlocking data — counts as 1, a discount that reflects how older nodes validate. A transaction's virtual size is its weight divided by 4, so a 400-weight-unit transaction is 100 vBytes. Dividing the 4,000,000 weight limit by 4 yields roughly 1,000,000 vBytes of room per block. Because witnesses are discounted, transaction types that push data into the witness — such as Taproot and native SegWit spends — tend to cost fewer vBytes than their legacy equivalents, which is a quiet argument for using modern address types.

How the fee is calculated

The total fee is simply fee rate multiplied by virtual size: a 140 vByte transaction at 10 sat/vB pays 1,400 satoshis. When the mempool is congested, the going rate climbs as transactions compete for space; when it is empty, even a 1 sat/vB transaction may confirm quickly. Wallets estimate a suitable rate from current mempool conditions, and a sovereign node runner can set the rate by hand rather than trusting a third party's guess. Fee estimation is inherently a prediction about the near future, so it is normal for two wallets to suggest different numbers for the same moment.

Bumping a stuck transaction

Because fee rate is only an estimate of demand, a transaction can be underpriced and left waiting. Two mechanisms fix this without abandoning it. Replace-By-Fee (RBF) rebroadcasts the same transaction at a higher rate, and Child-Pays-For-Parent (CPFP) spends the stuck transaction's output at a high rate so a miner must include both to collect the child's fee. Knowing these exist turns a stuck payment from a crisis into a routine adjustment.

Why it matters for self-custody

Fee rate is the one lever a spender fully controls. Overpay and you burn sats; underpay in a busy mempool and your transaction can sit unconfirmed for hours or days. Understanding sat/vB is therefore a core self-custody skill: it lets you consolidate coins cheaply during quiet periods, avoid overpaying during panics, and reason about when a low-fee transaction will eventually clear.

Reading a fee estimate well means understanding that block space is auctioned, not priced. Every roughly ten minutes a new block clears the highest-paying transactions from the queue, so the rate you need is really a bet on how much competition will arrive before the next few blocks are found. That is why fees can swing sharply within an hour: a burst of activity or a large consolidation can flood the mempool and push the clearing rate up, then collapse it again once the backlog drains. For a sovereign spender the practical wisdom is patience where possible. Non-urgent work such as consolidating small UTXOs is best done during quiet weekend hours when the going rate falls to a handful of sats per vByte, while genuinely time-sensitive payments justify paying the current market rate rather than gambling on a discount that may never come.

Fee rate is part of the miner's reward alongside the block subsidy; see coinbase maturity for how those rewards mature, inspect live fees in any block explorer, and watch congestion build in the mempool itself.