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Add 3 new topics: compactSize, Fee rate
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‎decoding/compact-size.mdx

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parent: "technical-foundation"
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---
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(coming soon)
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Compact Size (also known as CompactSize or var_int) is a variable-length integer encoding used in Bitcoin to efficiently represent numbers while minimizing space usage. It's primarily used in transaction data and network messages to indicate:
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- Number of inputs/outputs in a transaction
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- Script sizes
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- Number of witness elements
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- Length of upcoming data fields
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## Structure
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The encoding uses a prefix byte to determine how to read the subsequent bytes:
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<div className="overflow-x-auto">
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<table className="min-w-full bg-white dark:bg-gray-900 rounded-lg overflow-hidden">
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<thead className="bg-orange-100 dark:bg-orange-900">
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<tr>
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<th className="px-6 py-3 text-left text-sm font-semibold">Prefix</th>
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<th className="px-6 py-3 text-left text-sm font-semibold">Format</th>
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<th className="px-6 py-3 text-left text-sm font-semibold">Range</th>
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<th className="px-6 py-3 text-left text-sm font-semibold">Total Size</th>
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</tr>
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</thead>
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<tbody className="divide-y divide-gray-200 dark:divide-gray-800">
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<tr className="hover:bg-gray-100 dark:hover:bg-gray-800">
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<td className="px-6 py-4 whitespace-nowrap"><code>≤ 0xFC</code></td>
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<td className="px-6 py-4">Direct value</td>
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<td className="px-6 py-4">0-252</td>
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<td className="px-6 py-4">1 byte</td>
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</tr>
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<tr className="hover:bg-gray-100 dark:hover:bg-gray-800">
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<td className="px-6 py-4 whitespace-nowrap"><code>0xFD</code></td>
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<td className="px-6 py-4">Next 2 bytes (LE)</td>
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<td className="px-6 py-4">253-65,535</td>
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<td className="px-6 py-4">3 bytes</td>
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</tr>
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<tr className="hover:bg-gray-100 dark:hover:bg-gray-800">
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<td className="px-6 py-4 whitespace-nowrap"><code>0xFE</code></td>
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<td className="px-6 py-4">Next 4 bytes (LE)</td>
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<td className="px-6 py-4">65,536-4,294,967,295</td>
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<td className="px-6 py-4">5 bytes</td>
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</tr>
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<tr className="hover:bg-gray-100 dark:hover:bg-gray-800">
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<td className="px-6 py-4 whitespace-nowrap"><code>0xFF</code></td>
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<td className="px-6 py-4">Next 8 bytes (LE)</td>
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<td className="px-6 py-4">4,294,967,296-2^64-1</td>
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<td className="px-6 py-4">9 bytes</td>
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</tr>
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</tbody>
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</table>
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</div>
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### How to Read It
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Let's analyze real Bitcoin transactions to understand compact size encoding:
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1. **Transaction with 1 Input**
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-> <a href="https://mempool.space/tx/5e6e1a9b4ce3f9f39467d7129e3ecfbe6c81c08dd377aac666fedc9a758fe614" target="_blank">5e6e1a9b4ce3f9f...fedc9a758fe614</a>
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- Looking at input count: `01`
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- Since 1 < 252, it's a direct value
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- Meaning: Transaction has 1 input
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- Total bytes used: 1
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2. **ScriptSig of 107 bytes**
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-> <a href="https://mempool.space/tx/c27c4d2236fce2a7542e024408d7f89b95e50e42a2c3d10be499c3102ccb45ef" target="_blank">c27c4d2236fce2a...99c3102ccb45ef</a>
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- Looking at scriptsig size: `6b`
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- Since 107 (0x6b) < 252, it's a direct value
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- Meaning: ScriptSig is 107 bytes long
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- Total bytes used: 1
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3. **ScriptPubKey of 4,026 bytes**
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-> <a href="https://mempool.space/tx/e411dbebd2f7d64dafeef9b14b5c59ec60c36779d43f850e5e347abee1e1a455" target="_blank">e411dbebd2f7d64...347abee1e1a455</a>
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- Looking at scriptpubkey size: `fd ba 0f`
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- First byte is 0xFD, so read next 2 bytes
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- Next bytes: `ba 0f` (in little-endian)
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- Convert from little-endian: `0fba` = 4,026 in decimal
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- Meaning: ScriptPubKey is 4,026 bytes long
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- Total bytes used: 3
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<ExpandableAlert title="Understanding Little-Endian" type="info">
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In the third example, we see `ba 0f` instead of `0f ba` because of little-endian encoding.
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Think of it as reading the bytes from right to left: `0fba` is the actual number in hexadecimal.
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</ExpandableAlert>
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<ExpandableAlert title="Tip" type="warning">
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You can verify these values yourself by looking at the raw transaction data on the block explorer.
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Click the transaction IDs above and look for the "Raw Transaction" section.
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</ExpandableAlert>
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## Implementation
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Here's a Python implementation for reading and writing compact size integers:
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<CodeSnippet
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code={`def read_varint(s):
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'''Reads a variable integer from a stream
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The first byte determines the format:
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- If < 0xfd: directly contains the number
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- If 0xfd: next 2 bytes contain number
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- If 0xfe: next 4 bytes contain number
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- If 0xff: next 8 bytes contain number
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'''
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i = s.read(1)[0]
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if i == 0xfd:
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return int.from_bytes(s.read(2), 'little')
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elif i == 0xfe:
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return int.from_bytes(s.read(4), 'little')
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elif i == 0xff:
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return int.from_bytes(s.read(8), 'little')
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else:
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return i
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def encode_varint(i):
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'''Encodes an integer as a compact size
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Returns bytes object with the encoded number
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'''
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if i < 0xfd:
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return bytes([i])
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elif i < 0x10000:
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return b'\\xfd' + i.to_bytes(2, 'little')
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elif i < 0x100000000:
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return b'\\xfe' + i.to_bytes(4, 'little')
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else:
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return b'\\xff' + i.to_bytes(8, 'little')`}
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language="python"
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/>
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## Common Uses in Bitcoin
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1. **Transaction Structure**
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- Input count
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- Output count
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- ScriptSig size
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- ScriptPubKey size
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- Witness element count
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<TransactionCreation enabledFields={["input_count", "output_count", "scriptsig_size", "scriptpubkey_size"]} />
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2. **Block Data**
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- Number of transactions in a block
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3. **Network Messages**
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- Length of upcoming message data
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- Number of inventory items
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<ExpandableAlert title="Warning" type="warning">
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The compact size format is part of Bitcoin's consensus rules. Incorrect encoding
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or decoding can lead to invalid transactions or network messages.
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</ExpandableAlert>
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<ExpandableAlert title="Important Notes" type="info">
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- Most transactions use single-byte encoding (≤ 252)
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- The FF prefix (8-byte numbers) is rarely used in practice
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- All multi-byte numbers must be encoded in little-endian format
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- This encoding has been part of Bitcoin since its first release
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</ExpandableAlert>
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This variable-length integer encoding plays a crucial role in keeping transaction and block data compact while maintaining flexibility for larger values when needed.

‎decoding/endianness.mdx

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Little-endian stores the **least significant byte** first. This might feel counterintuitive to humans but is more efficient for modern processors.
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Using the same number **12345678** (`0x00BC614E`), here’s how it looks in **little-endian**:
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Using the same number **12345678** (`0x00BC614E`), here's how it looks in **little-endian**:
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<CodeSnippet
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language="Hex"
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This duality requires developers to frequently convert between the two formats when working with Bitcoin data.
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---
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## 4. Working with Endianness in Code
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When working with Bitcoin data, you'll frequently need to convert between little-endian and big-endian formats. Here are some common Python functions for handling endianness:
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### Little-Endian to Integer
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<CodeSnippet
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language="python"
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code={`def little_endian_to_int(b):
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'''Convert little-endian bytes to integer'''
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return int.from_bytes(b, 'little')
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# Example usage
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bytes_data = bytes.fromhex('4E61BC00') # 12345678 in little-endian
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number = little_endian_to_int(bytes_data)
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print(number) # Output: 12345678`}
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/>
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### Integer to Little-Endian
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<CodeSnippet
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language="python"
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code={`def int_to_little_endian(n, length):
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'''Convert integer to little-endian bytes of specified length'''
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return n.to_bytes(length, 'little')
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# Example usage
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number = 12345678
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bytes_data = int_to_little_endian(number, 4)
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print(bytes_data.hex()) # Output: 4e61bc00`}
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/>
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### Common Gotchas
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1. **Byte Order Confusion**: When reading transaction IDs or hashes:
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<CodeSnippet
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language="python"
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code={`# INCORRECT - reading txid directly from hex
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txid = "4e61bc0000000000000000000000000000000000000000000000000000000000"
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# CORRECT - reverse the bytes for proper txid display
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txid_bytes = bytes.fromhex(txid)
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txid_display = txid_bytes[::-1].hex()`}
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/>
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2. **Length Specification**: When converting to little-endian, always specify the correct byte length:
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<CodeSnippet
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language="python"
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code={`# For Bitcoin amounts (8 bytes)
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amount = int_to_little_endian(12345678, 8) # Correct
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amount = int_to_little_endian(12345678, 4) # Wrong - too short!`}
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/>
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---

‎decoding/fee-rate.mdx

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parent: "fee-calculation"
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---
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(coming soon)
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When miners select transactions for inclusion in a block, they don't just look at the raw fee amount, they evaluate transactions based on their _fee rate_.
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This is how we calculate the fee rate:
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<div className="text-center my-4 text-orange-500">
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$$\mathbf{Fee\ Rate = \frac{Transaction\ Fee}{Transaction\ Size}}$$
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</div>
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Miners use the fee rate to maximize their profit by comparing the profitability of including different transactions in their blocks.
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For example, consider these two transactions:
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1. Transaction A: 50,000 sats fee, 250 vbytes size
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2. Transaction B: 30,000 sats fee, 100 vbytes size
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**Fee Rates:**
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- Transaction A: 200 sats/vbyte (50,000 ÷ 250)
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- Transaction B: 300 sats/vbyte (30,000 ÷ 100)
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<ExpandableAlert title="Fee Rate Priority" type="info">
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Even though Transaction A pays a higher absolute fee, Transaction B is more profitable for miners because it provides more fees per unit of block space.
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</ExpandableAlert>
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## Fee Rate Units
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There are three common ways to measure fee rates:
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1. **sats/vbyte (Most Common)**
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2. **sats/wu (Internal)**
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3. **sats/byte (Legacy)**
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<div className="overflow-x-auto">
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<table className="min-w-full bg-white dark:bg-gray-900 rounded-lg overflow-hidden">
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<thead className="bg-orange-100 dark:bg-orange-900">
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<tr>
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<th className="px-6 py-3 text-left text-sm font-semibold">Unit</th>
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<th className="px-6 py-3 text-left text-sm font-semibold">Description</th>
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<th className="px-6 py-3 text-left text-sm font-semibold">Usage</th>
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<th className="px-6 py-3 text-left text-sm font-semibold">Example</th>
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</tr>
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</thead>
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<tbody className="divide-y divide-gray-200 dark:divide-gray-800">
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<tr className="hover:bg-gray-100 dark:hover:bg-gray-800">
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<td className="px-6 py-4 whitespace-nowrap font-semibold"><code>sats/vbyte</code></td>
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<td className="px-6 py-4">Virtual bytes account for witness discount</td>
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<td className="px-6 py-4">Most common unit used by wallets and block explorers</td>
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<td className="px-6 py-4">50 sats/vbyte</td>
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</tr>
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<tr className="hover:bg-gray-100 dark:hover:bg-gray-800">
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<td className="px-6 py-4 whitespace-nowrap font-semibold"><code>sats/wu</code></td>
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<td className="px-6 py-4">Weight units used internally by Bitcoin Core</td>
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<td className="px-6 py-4">Internal Bitcoin Core (4M WU block limit)</td>
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<td className="px-6 py-4">12.5 sats/wu</td>
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</tr>
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<tr className="hover:bg-gray-100 dark:hover:bg-gray-800">
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<td className="px-6 py-4 whitespace-nowrap font-semibold"><code>sats/byte</code></td>
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<td className="px-6 py-4">Deprecated since SegWit activation</td>
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<td className="px-6 py-4">Legacy software only</td>
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<td className="px-6 py-4">50 sats/byte</td>
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</tr>
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</tbody>
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</table>
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</div>
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## Understanding Virtual Bytes (vBytes)
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Virtual bytes (vBytes) are a way to express transaction size that accounts for the SegWit discount. The formula is simple:
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<div className="text-center my-4 text-orange-500">
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$$\mathbf{Virtual\ Bytes = Transaction\ Weight ÷ 4}$$
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</div>
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<div className="overflow-x-auto">
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<table className="min-w-full bg-white dark:bg-gray-900 rounded-lg overflow-hidden">
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<thead className="bg-orange-100 dark:bg-orange-900">
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<tr>
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<th className="px-6 py-3 text-left text-sm font-semibold">Transaction Type</th>
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<th className="px-6 py-3 text-left text-sm font-semibold">Weight Units</th>
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<th className="px-6 py-3 text-left text-sm font-semibold">Virtual Bytes</th>
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</tr>
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</thead>
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<tbody className="divide-y divide-gray-200 dark:divide-gray-800">
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<tr className="hover:bg-gray-100 dark:hover:bg-gray-800">
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<td className="px-6 py-4">Legacy (non-SegWit)</td>
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<td className="px-6 py-4">size × 4</td>
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<td className="px-6 py-4">Same as raw size</td>
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</tr>
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<tr className="hover:bg-gray-100 dark:hover:bg-gray-800">
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<td className="px-6 py-4">Native SegWit</td>
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<td className="px-6 py-4">(base × 4) + witness</td>
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<td className="px-6 py-4">~25% smaller than raw size</td>
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</tr>
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</tbody>
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</table>
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</div>
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<ExpandableAlert title="Example Calculation" type="info" expandable={true} initialLines={2}>
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Consider a SegWit transaction with:
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- Base size: 200 bytes
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- Witness size: 100 bytes
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1. Weight = (200 × 4) + 100 = 900 weight units
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2. Virtual Size = 900 ÷ 4 = 225 vBytes
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Even though the raw size is 300 bytes (200 + 100), the virtual size is only 225 vBytes thanks to the SegWit discount!
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</ExpandableAlert>
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## Minimum Relay Fee Rate
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Nodes enforce a minimum fee rate to prevent spam:
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- Default minimum: 1 sat/vbyte
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- Transactions below this rate are typically rejected
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- You can check your node's setting:
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<CodeSnippet
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language="bash"
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code={`$ bitcoin-cli getmempoolinfo
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{
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"loaded": true,
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"size": 47293,
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"bytes": 32883821,
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"usage": 148075552,
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"maxmempool": 300000000,
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"mempoolminfee": 0.00001000, # 1 sat/vbyte
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...
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}`}
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/>
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<ExpandableAlert title="NOTE" type="info">
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💡 The minimum relay fee is a policy rule, not a consensus rule. While
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theoretically possible, getting a below-minimum fee transaction mined is
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extremely unlikely unless you have a direct connection to a miner.
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</ExpandableAlert>
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## RPC: Current Fee Rates
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You can estimate current fee rates using Bitcoin Core:
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<CodeSnippet
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language="bash"
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code={`$ bitcoin-cli estimatesmartfee 6
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{
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"feerate": 0.00008124, # BTC/kB
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"blocks": 6
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}`}
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/>
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Or query popular block explorers:
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- mempool.space
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- blockstream.info
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- bitcoinfees.earn.com
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Remember that fee rates are dynamic and can change rapidly based on network demand.

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