Transaction Size

An introduction to calculating the size of a transaction, which is used to determine the transaction fee.

Transaction size is usually measured in bytes and is determined by calculating the size of the transaction's inputs, outputs, and other fields.

Components of Transaction Size

A Bitcoin transaction consists of the following parts:

1. Version Number

  • Size: 4 bytes

  • Purpose: Indicates the transaction format and protocol version.

2. Input Counter

  • Size: 1-9 bytes (variable-length integer)

  • Purpose: Indicates the number of inputs included in the transaction.

3. Inputs

Each input consists of the following fields:

  • Previous Transaction Hash: 32 bytes

  • Previous Output Index: 4 bytes

  • Unlocking Script Length (scriptSig length): 1-9 bytes (variable-length integer)

  • Unlocking Script (scriptSig): Typically 107 bytes (usual size of a signature and public key)

  • Sequence Number: 4 bytes

4. Output Counter

  • Size: 1-9 bytes (variable-length integer)

  • Purpose: Indicates the number of outputs included in the transaction.

5. Outputs

Each output consists of the following fields:

  • Value: 8 bytes

  • Locking Script Length (scriptPubKey length): 1-9 bytes (variable-length integer)

  • Locking Script (scriptPubKey): Typically 25 bytes (usual size for P2PKH)

6. Lock Time

  • Size: 4 bytes

  • Purpose: Specifies the time or block height when the transaction can be added to the blockchain.

Steps to Calculate Transaction Size

1. Calculate the Size of Fixed Fields

  • Version Number: 4 bytes

  • Lock Time: 4 bytes

2. Calculate the Size of Input and Output Counters

Assuming we have n inputs and m outputs, the size of the input and output counters depends on the values of n and m, typically 1 byte each.

3. Calculate the Size of All Inputs

Each input size is:

Previous Transaction Hash (32 bytes) + Previous Output Index (4 bytes) + Unlocking Script Length (1-9 bytes) + Unlocking Script (approximately 107 bytes) + Sequence Number (4 bytes)

4. Calculate the Size of All Outputs

Each output size is:

Value (8 bytes) + Locking Script Length (1-9 bytes) + Locking Script (approximately 25 bytes)

5. Calculate Total Transaction Size

Add the sizes of the above components to get the total transaction size.

Example

Assume a transaction with 2 inputs and 2 outputs:

  • Version Number: 4 bytes

  • Input Counter: 1 byte

  • Each Input: 148 bytes

  • Output Counter: 1 byte

  • Each Output: 34 bytes

  • Lock Time: 4 bytes

Calculation:

Total Size = 4 (Version Number) + 1 (Input Counter) + 2 * 148 (Inputs) + 1 (Output Counter) + 2 * 34 (Outputs) + 4 (Lock Time)
Total Size = 4 + 1 + 296 + 1 + 68 + 4 = 374 bytes

By following these steps, you can accurately calculate the size of a Bitcoin transaction. This is crucial for setting appropriate fees and ensuring the transaction is confirmed quickly.

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