About the Octal to Decimal Converter
Octal numbers use only the digits 0 to 7. You still see them in Unix permission codes such as 755, in some old computer documentation and in escape sequences. This converter turns octal into ordinary decimal. Paste one octal number per line, with or without a 0o prefix, and the decimal values appear on the right. A digit 8 or 9 is not valid octal, so such lines are flagged with a warning instead of being converted wrongly. Large numbers are exact and negative numbers keep their sign. The conversion runs in your browser.
Updated October 2026
How to use the Octal to Decimal Converter
- 1Type or paste octal numbers, one per line.
- 2A 0o prefix, spaces and underscores are accepted.
- 3Read the decimal values on the right.
- 4Check the note for lines that could not be read.
- 5Copy the result or download it.
Reading octal
Each octal digit stands for three binary digits, which is why it is natural for permissions: three flags make one digit. A permission of 7 means all three flags are on, 5 means read and execute and 4 means read only. Converting the whole number to decimal is rarely needed for chmod but often needed for programming, tests and documentation.
Older systems such as some minicomputers used octal throughout, so historic listings are full of it.
Worked examples
Octal 17 is 15 because 1 times 8 plus 7 is 15. Octal 7777 is 4095. Octal 644 is 420. Octal 0o644 is the same number, with the Python-style prefix on the front.
- 17 → 15
- 644 → 420
- 755 → 493
- 7777 → 4095
Next steps
To convert back, use the Decimal to Octal Converter. For hex, use the Hex to Octal Converter. If the octal represents text bytes, the Octal to Text Converter decodes them into words.
Using it on phone and computer
Paste octal numbers, one per line, into the left box and the result appears straight away, so there is no button to press. On a phone, long-press the box to paste and use the Copy button to move the result to another app. On a computer you can also download it as a text file. Students use it to check homework on number bases, and admins use it to compare permission values numerically.
Accuracy and limits
As a check, 10 in octal is 8 and 100 in octal is 64. The tool works with exact whole-number arithmetic rather than floating point, so long values are not rounded. Anything it cannot read is shown with a warning that names the line, and the rest of your list is still converted. Nothing you paste is uploaded or stored, and closing the tab removes it.
Features
- Many numbers at once
- Accepts the 0o prefix
- Flags digits 8 and 9
- Exact for very large numbers
- Negative numbers supported
- Private: nothing is uploaded
Frequently asked questions
How do I convert octal to decimal?
Multiply each digit by 8 to the power of its position, counting from the right, and add. For 755 that is 7 times 64, plus 5 times 8, plus 5, which is 493.
What is 755 in decimal?
493. In Unix it is the permission set rwxr-xr-x: full rights for the owner, read and execute for others. 644 is 420 in decimal.
Why does the tool reject 8 and 9?
Octal has only eight digits, 0 to 7, so 8 and 9 do not exist in it. A line with such a digit is probably decimal already, and the warning tells you that.
What does a leading zero mean?
In C-style code a leading zero marks an octal number, so 0755 is octal. The tool reads such numbers correctly, and ignores leading zeros in the result.
Can it convert very large octal numbers?
Yes. The conversion is exact for any length, so you won't see rounding errors on long values.
Is my data sent anywhere?
No. All conversion is done in your browser, and nothing is uploaded.