Octal to Decimal Converter: base 8 to base 10 online
Free octal to decimal converter that runs in your browser. Translate base 8 numbers and Unix chmod permissions to the decimal integers APIs and file systems expect — with batch input and 0o prefix support.
Octal in, decimal out
Octal groups 3 binary bits per digit. The classic Unix permission code 644 means decimal 420 (the integer file mode some APIs require).
10 100 644
8 64 420
What you'll use this for
Octal lingers in Unix permissions, file modes, and a handful of legacy systems where 3-bit groupings made hardware sense.
Unix permissions
Translate chmod codes to their integer values.
Legacy compatibility
Old C constants and config files sometimes use octal literals.
Reading old memory dumps
Some historical systems wrote machine words in 3-bit octal groups.
CS homework
Quickly verify base-conversion exercises and bit-twiddling answers.
How to convert octal to decimal
Paste octal numbers
One per line, or separated by spaces or commas. 0o prefixes are stripped automatically.
Choose strict mode
Strict rejects tokens containing 8 or 9; loose simply ignores those characters.
Click Convert
Or leave auto-convert on for live updates. Runs locally — no upload.
Copy or download
Copy to clipboard or save as .txt. Round-trip via the reverse tool to verify.
Frequently asked questions
Mostly Unix permissions (chmod 755). Some legacy APIs and file modes.
0–7. Strict mode rejects 8/9; loose mode strips them.
Yes.
It's stripped automatically.
About octal-to-decimal conversion
Octal (base 8) uses the digits 0-7. It survived in Unix because each octal digit maps cleanly onto exactly three permission bits — read, write, execute — for one user class.
File permission mapping
7= 111 (rwx — all)6= 110 (rw-)5= 101 (r-x)4= 100 (r--)0= 000 (---)
So chmod 644 means owner can rw, group and others can read — and as a decimal integer file mode it's 420.
How this converter works
- Splits input on whitespace and commas, strips any
0oprefix. - Parses each token with
parseInt(t, 8)and emits decimal viatoString(10). - Strict mode raises an error on stray 8 or 9 digits; loose mode silently ignores them.