A Bitcoin wallet quoting 4 sat/vB has not told you the total transaction fee. That number is a rate per unit of transaction size. To work out the cost, you also need the transaction’s virtual size. Two payments made at the same rate can therefore carry different fees, even before you convert either cost into euros.
| What you need to know |
|---|
| Multiply the rate in sat/vB by the virtual size in vB to estimate the fee in satoshis. |
| A 250-vB transaction at 4 sat/vB costs 1,000 satoshis in the worked example. |
| Bitcoin Core documents sat/vB and BTC/kvB as different input units. |
| A higher fee can improve a transaction's prospects; it does not promise a confirmation time. |
Read the rate and the total separately
A satoshi is one hundred-millionth of a bitcoin. The abbreviation vB means virtual byte, Bitcoin’s size measure for fee calculation. Bitcoin Core’s transaction decoder documentation distinguishes virtual size from the transaction’s ordinary serialized size. Use the wallet’s virtual-size estimate rather than the size of a downloaded file.
Take a hypothetical transaction estimated at 250 vB and a chosen rate of 4 sat/vB. Multiplication gives 250 × 4 = 1,000 satoshis, or 0.00001 BTC. If the virtual size is 500 vB at the same rate, the fee becomes 2,000 satoshis, or 0.00002 BTC. These are illustrative inputs, not measurements of current network demand or a recommended fee.
The calculation explains why the rate alone is insufficient. Nor does it tell you that a larger bitcoin payment must cost more: the fee calculation uses transaction size, rather than a percentage of the amount sent. Inputs and outputs matter when a wallet builds the transaction. Check its proposed total before signing instead of estimating the cost from the payment amount.
A similar label can hide a different unit
The Bitcoin Core 30.0 funding documentation provides a concrete warning for anyone comparing tools. Its fee_rate option takes sat/vB; the similarly named feeRate takes BTC/kvB. A kilovirtual byte is 1,000 vB. One sat/vB therefore equals 0.00001 BTC/kvB, and the example’s 4 sat/vB equals 0.00004 BTC/kvB.
Those labels belong to the documented interface and version; they are not a universal naming rule for every wallet. Read the unit displayed by your own tool. Copying the same numerical value between fields with different units does not preserve the intended rate.
Even an explicit rate is not always the complete explanation of the final quote. Core’s documentation says funding a transaction with unconfirmed inputs can produce a higher resulting rate because the wallet aims to meet the selected rate for the whole transaction package. Treat the simple multiplication as a worked calculation for the stated size and rate, then inspect the actual transaction the wallet proposes.
If the payment remains unconfirmed
Electrum’s official FAQ explains that confirmation depends on miners including a transaction and describes dynamic fee estimates. A wallet’s estimate or target is not a booking for a particular block.
Electrum describes increasing the fee for a replaceable transaction and, in suitable cases, creating a child transaction that pays for its parent. These are additional transactions or replacements with their own conditions and costs. Check the existing transaction’s details and your wallet’s documented options before acting; the FAQ does not establish that every wallet offers the same controls.
For a euro budget, keep the two conversions separate: first calculate satoshis from the transaction’s size and rate, then value that bitcoin amount using the exchange rate you are actually offered. The original quote can change while the wallet rebuilds a transaction. The amount to approve is the final displayed fee, with its unit and transaction details checked together.
Technical documentation checked on 12 October 2026 at 00:18 CEST. No live fee or bitcoin-price observation is asserted. The image is a conceptual illustration of size measurement with an authentic Bitcoin logo, not transaction evidence.
