Data Capacity & vBytes

In the early days of Bitcoin, a byte was just a byte. If your transaction was 200 bytes, you paid for 200 bytes. However, with the activation of SegWit, the relationship between a physical byte and its "Cost" changed.

1. Physical Bytes vs. Virtual Bytes

When you save a transaction to a hard drive, it takes up Physical Bytes. When the Bitcoin network calculates your fee, it uses Virtual Bytes (vBytes).

• Base Data: 1 Physical Byte = 1.0 vByte.

• Witness Data: 1 Physical Byte = 0.25 vByte.

2. Why the distinction?

The network wants to encourage users to use the "Witness" area (for signatures) because that data doesn't need to be kept in the UTXO set forever.

• By making "Witness Bytes" cheaper, the network makes complex transactions (like multisig) more affordable.

• It also allows the block size to grow beyond 1MB without breaking old nodes.

3. The 1MB Legacy Limit

Old nodes still only accept blocks that are 1,000,000 physical bytes in the "Base" area.

• New nodes see the base area PLUS the witness area.

• A SegWit block might be 2.5MB in physical size, but as far as an old node is concerned, the "Stripped" version is still under 1MB.

4. Measuring "Efficiency"

A transaction's efficiency is often measured by its vSize to Weight ratio.

• Weight: Measured in Weight Units (WU). 1 vByte = 4 WU.

• Max Block Weight: 4,000,000 WU.

5. The Byte-Cost of Privacy

Every byte you add to a transaction increases your fee.

• Privacy cost: Adding extra outputs (for change or decoys) costs bytes.

• Complexity cost: Adding more signers to a multisig costs bytes. Understanding the "Byte Economy" is essential for designing Bitcoin wallets that are both private and cost-effective.

Transaction Type	Physical Size	Virtual Size (vSize)
Legacy (P2PKH)	226 bytes	226 vBytes
Native SegWit	222 bytes	~141 vBytes

In the next section, we will analyze Raw Byte Mapping (ASCII vs. Binary).