Transaction Weight

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Fee Rate
Fee Bumping (RBF, CPFP)
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When sending Bitcoin transactions, we need to pay fees based on how much block space our transaction occupies. There are three main ways to measure a transaction's size:

Transaction Size: The raw byte count of the transaction
Transaction Weight: A weighted measure that counts witness data differently
Virtual Bytes (vBytes): Used for fee calculations (covered in the next topic on fee rates)

1- Transaction Size

The most straightforward measurement is the raw byte count of a transaction:

Transaction Size = Bytes of serialized transaction

For example, here's a raw transaction in hexadecimal:

This transaction is 225 bytes long - we simply count the number of characters and divide by 2 (since each byte is represented by 2 hex characters).

2- Transaction Weight

With the introduction of SegWit, a new measurement called "transaction weight" was created:

Transaction Weight = (Base Size × 4) + Witness Size

Where:

Base Size = Size of transaction data without witness data (signatures)
Witness Size = Size of witness data (signatures) only

💡 For legacy (non-SegWit) transactions, all data is considered base size and witness size is zero.

Read more about fee calculation →]https://bitcoindevs.xyz/decoding/fee-calculation)