Octal to Decimal Converter
Convert octal (base-8) numbers to decimal (base-10). Supports Unix permission values, 0o-prefixed octal, and 32-bit two's complement interpretation for negative values.
Conversion Options
How to interpret large octal values as negative numbers
Recommended Settings
Pro Tips
- •Octal uses digits 0-7; anything with an 8 or 9 is not valid octal
- •0o and leading-0 prefixes are automatically stripped before conversion
- •Unix permission values like 644 or 755 convert directly to their decimal equivalent
- •Two's complement mode interprets large 32-bit octal values (like 37777777766) as negative numbers
- •Multiple values can be converted at once - one per line, or separated by spaces or commas
Most Popular
Most users convert Unix permission values or leave two's complement handling on its default
When to Use This Tool
Understand exactly what decimal value a chmod octal mode like 644 or 755 corresponds to, useful when working with APIs or tools that expect permissions as plain integers.
Convert octal register values, memory offsets, or hardware documentation values into decimal for use in code, debuggers, or documentation.
Decode octal values found in legacy Unix systems, shell scripts, or system configuration files where octal notation is still used for file modes and device numbers.
Interpret large 32-bit octal values that represent negative numbers in two's complement form, common when inspecting raw register or memory dumps.
How It Works
Parse input octal numbers from text (separated by spaces, commas, or newlines)
Strip 0o and leading-0 prefixes, and validate that only digits 0-7 remain
Convert each octal string to its decimal value using base-8 positional notation
Apply negative number handling: literal sign, two's complement, or unsigned interpretation
Optionally format the result with thousands separators for readability
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Frequently Asked Questions
How does octal to decimal conversion work?
Each octal digit is multiplied by a power of 8 based on its position, starting from 8^0 on the right. For example, octal 17 = 1×8^1 + 7×8^0 = 8+7 = 15 in decimal.
How do Unix file permissions convert to decimal?
Each octal digit (owner, group, others) is the sum of read=4, write=2, execute=1. Converting the whole octal number to decimal gives you the raw integer some APIs and tools expect - for example, octal 644 equals decimal 420.
What's the difference between the negative number handling modes?
'Two's Complement' treats large 32-bit octal values (above 17777777777) as negative numbers, matching how many systems represent negative integers. 'Negative Sign Only' respects a literal minus sign in the input. 'Unsigned' always returns a positive value, ignoring any sign interpretation.
What happens with 0o or leading-zero prefixes?
Both the modern 0o prefix (e.g., 0o755) and the classic C-style leading zero (e.g., 0755) are automatically stripped before conversion, so you can paste octal values in either style.
Can I convert multiple octal numbers at once?
Yes. Enter numbers one per line, or separate them with spaces or commas, and every valid number will be converted in a single pass.