Decimal to Octal Converter

What this tool is used for

• Producing a numeric file-permission value for a chmod command from a computed number.
• Converting a decimal value for a system or a protocol that specifies octal.
• Checking a permission value's three-digit octal form against the bits you intended.
• Producing an octal escape for a string literal.
• Working through a base-conversion exercise where base eight groups bits in threes.

Frequently Asked Questions

How do I convert by hand?

Divide by 8 repeatedly and read the remainders bottom-up: 255 ÷ 8 = 31 r 7, 31 ÷ 8 = 3 r 7, so 377. Alternatively convert to binary and group in threes, which is faster and less error-prone.

Why does a byte max out at 377?

Because 8 bits split into three-bit groups need three octal digits covering 9 bits, so the leading digit uses only 2 of them and ranges 0-3. Any octal byte value starting with 4-7 is out of range.

Where will I actually use this?

chmod permissions almost exclusively — 755, 644, 600. Each digit is one three-bit rwx field, which is exactly why permissions are quoted in octal rather than decimal or hex.

What does a leading zero mean in code?

In C, Java and older JavaScript it makes the literal OCTAL, so 0755 is 493, not 755. This has caused real bugs with zero-padded values. Python 3 rejects it outright and requires the explicit 0o prefix.

Why did octal lose to hexadecimal?

Word size. Octal groups three bits, which suited 12- and 36-bit machines like the PDP-8; hex groups four, which divides an 8-bit byte evenly. Once bytes became universal, hex was the natural fit.

How do I read a Unix permission value?

Each digit is one triple of read, write and execute — owner, group, others. So 755 is `rwxr-xr-x`, and the fourth leading digit, when present, carries setuid, setgid and the sticky bit.

What is the umask and why is it octal too?

A mask of permission bits to REMOVE from a new file's default, so 022 clears group and other write. It is octal for the same reason permissions are — three bits per digit maps exactly onto the triples.

Common errors and gotchas

• Forgetting the leading zero many contexts use to mark a value as octal, which changes how it is read.
• Producing more than three digits for a permission value, which no longer means what chmod expects.
• Confusing the octal form with the decimal one, where 755 and 493 are the same permissions written differently.
• Assuming the tool adds a prefix, when some targets require one and others treat it as part of the number.
• Converting a value above the field's range and getting a digit count the target silently truncates.