Octal to Decimal Converter

What this tool is used for

• Turning a file-permission value like 755 into the decimal number a system call wants.
• Reading a legacy value quoted in octal into modern decimal terms.
• Checking an octal escape's numeric value while reading source code.
• Converting a device or inode number reported in octal.
• Working through a base-conversion exercise with base eight.

Frequently Asked Questions

Where is octal still used?

Almost entirely in Unix file permissions — chmod 755 is octal — because three permission bits (read, write, execute) map exactly onto one octal digit. Outside that, hexadecimal replaced it decades ago.

How do I read a chmod value?

Each digit is owner, group, other; within a digit 4 = read, 2 = write, 1 = execute, summed. So 755 is rwx for owner and r-x for the rest, and 644 is rw- then r--. The optional fourth leading digit carries setuid, setgid and sticky.

Why does a leading zero change a number in C?

Because 0755 is an octal literal, not decimal 755 — it means 493. This has caused real bugs in date handling and zero-padded IDs. Python 3 forbids it outright and requires the explicit 0o prefix.

How does octal relate to binary?

One octal digit is exactly three bits, so conversion is pure grouping with no arithmetic: 111101 splits into 111 101 = 75. Hex groups four bits at a time, which is why hex won once byte-oriented (8-bit) machines displaced 12- and 36-bit ones.

Why was octal popular historically?

Early machines like the PDP-8 used 12-bit and 36-bit words, which divide evenly by three but not by four — so octal grouped their bits cleanly. The move to 8-bit bytes made hex the natural fit instead.

How do I convert by hand?

Multiply each digit by the corresponding power of eight and sum — 755 is 7×64 + 5×8 + 5, which is 493. It is the same positional method as any base, with 8 in place of 10.

Why do permission values look like decimal numbers?

Because 755 and 644 are valid in both bases and look unremarkable, which is exactly the trap — read as decimal they are meaningless, and a config file storing one as a decimal integer silently applies the wrong permissions.

Common errors and gotchas

• Reading the octal digits as decimal, so 755 is taken as seven hundred and fifty-five rather than 493.
• Overlooking an 8 or a 9, which proves the input was never octal.
• Treating the leading zero as part of the number rather than the marker that it is octal.
• Assuming three digits is always the whole value, when permissions can carry a fourth for setuid bits.
• Converting each digit independently rather than by place value.