Octal to Decimal Converter — Free Online Base 8 to Base 10

Octal (base 8) uses digits 0–7 and was once common in early computing systems. Today its primary surviving use is Unix/Linux file permissions, where 3-bit groups map naturally to one octal digit. Understanding octal-to-decimal conversion is essential for any sysadmin or Linux developer.

Each octal digit represents 3 binary bits (since 2^3 = 8). The permission string rwxr-xr-x breaks into three groups of 3: rwx=7, r-x=5, r-x=5 → octal 755 → decimal 493. The conversion algorithm is the same as any other base: multiply each digit by its power of 8 from right to left.

In C programming, octal literals start with 0 (e.g., 0755), which is why accidentally writing 0123 instead of 123 in C gives you a different value (83 instead of 123). This is a common source of bugs. Python 3 made this explicit with the0o prefix to avoid confusion.