Virtual Size (vSize)

While miners use Weight Units for consensus, humans and wallets typically use Virtual Size (vSize) for fee estimation. vSize is a "normalized" byte count that makes SegWit transactions comparable to legacy ones.

1. The vSize Formula

To convert Weight Units back into a byte-like number, we divide by 4:

vSize = ceil(Weight / 4)

The "ceiling" function means we always round up to the nearest whole number.

2. Why do we need vSize?

If a wallet says a fee is "10 sats/byte," but you are using SegWit, which "byte" is it referring to?

• If it refers to raw bytes, the calculation is wrong because signatures are discounted.

• If it refers to weight units, the number is too small (e.g., 2.5 sats/WU). vSize solves this by providing a "Virtual Byte" count that behaves exactly like legacy bytes for fee purposes.

3. Calculation Example

Take a transaction with:

• 160 bytes of Base Data

• 100 bytes of Witness Data

Step 1: Calculate Weight (160 * 4) + (100 * 1) = 740 WU

Step 2: Calculate vSize 740 / 4 = 185 vBytes

Step 3: Compare to Raw Size The actual physical size is 160 + 100 = 260 bytes. The transaction only "pays" for 185 bytes. This is the SegWit efficiency in action.

4. Summary of Units

Unit	Used For	Context
Weight Units	Block Limits	Consensus / Mining
Raw Bytes	Network Traffic	Bandwidth / Storage
vSize	Fee Estimation	Wallets / Users

In the next section, we will discuss Stripped Size vs. Total Size.