About the Octal to Decimal
Octal is base 8 and uses only the digits 0–7. Each position is worth eight times the one to its right: 1, 8, 64, 512, 4096 … To convert to decimal, multiply each digit by its power of 8 and add. For 755: 7×64 + 5×8 + 5×1 = 448 + 40 + 5 = 493. The step table under the result shows this for your number.
Where you meet octal today is mainly Unix file permissions. In chmod 755 each digit is three bits — read (4), write (2), execute (1) — for owner, group and others, so 7 = rwx and 5 = r-x. The decimal value (493) is what a program receives if it parses the mode as a plain number, a classic bug when calling os.chmod(path, 755) in Python instead of 0o755.
Watch out for implicit octal in code. In C, C++, Java and older JavaScript, a literal with a leading zero is octal: 010 equals 8, not 10. Python 3 requires the explicit 0o prefix and treats 010 as a syntax error; JavaScript also forbids 010 in strict mode and modules. This converter accepts 0o prefixes and rejects the digits 8 and 9, which cannot appear in an octal number.
How to use it
- Type the octal number (digits 0–7; a 0o prefix is fine).
- Read the decimal result.
- Follow the step-by-step table showing each digit times its power of 8.
- Copy the result; binary and hex are listed under “Other bases”.