Decimal to Octal Converter

Q: Can I convert many decimal numbers at once?

Yes. Put one number per line and every line is converted, with the results lined up against the input. A line that is not a valid decimal number is left blank and reported by line number, so one typo does not stop the rest.

Q: How do you convert decimal to octal?

Divide by 8 repeatedly and write down the remainders, then read them from last to first. 100 → 12 r4, 12 → 1 r4, 1 → 0 r1, so 144.

Q: Why does my file mode show as 33261?

Because stat() includes the file-type bits. 33261 in octal is 100755: the 100 means a regular file and 755 is the permission. Convert it here and read the last three digits.

Q: Why is 0755 dangerous in code?

In C, older JavaScript and some config formats a leading zero makes a number octal, so 0755 is 493. In other contexts it is seven hundred fifty-five. Writing 0o755 removes the ambiguity where the language supports it.

Convert decimal to octal instantly, with 0o prefixes and Unix permission readings for values up to 7777.

Decimal

Octal

Group digits Add 0x / 0b / 0o prefix Uppercase hex Pad to whole bytes

What It Does

This converter writes a decimal number in octal (base 8). It is exact at any size and can add the 0o prefix. The most common reason to need octal today is file permissions, where a mode stored as a decimal integer — as some APIs and languages return it — has to be turned back into the familiar three digits; the tool shows the rwx reading for those values.

When to Use It

An API or stat() call returned a file mode as a decimal number and you need the octal permission.
You are writing an octal literal or escape sequence in code.
You are working through base-conversion exercises.
You are reading values from a legacy system that uses octal.

Worked Examples

493

755 in octal — the decimal that Node's fs.stat reports for an rwxr-xr-x file once the file-type bits are masked off.

420

644 — rw-r--r--. A decimal 420 in a config file is often a mangled 0644.

0755

Read as decimal seven hundred fifty-five, giving 1363 — and a warning, because many languages would treat 0755 as octal.

Features

Converts as you type

Bulk conversion one number per line

Exact for numbers of any size

Digit grouping and 0x/0b/0o prefixes

Swap direction in one click

Explains bit width and two's complement

How to Use

1. Type or paste a decimal number — or a list, one per line. 2. The octal value appears alongside, line for line. 3. Use the options to group digits or add a prefix, and the swap button to convert the other way.

Common Mistakes

Converting a number through a JavaScript Number. Above 9,007,199,254,740,991 it silently rounds, so the last digits come out wrong. This tool uses BigInt and stays exact at any size.
Forgetting that the same digits mean different values in different bases. 10 is ten in decimal, two in binary, eight in octal and sixteen in hex — always label which base a value is in.
Treating a minus sign as how a computer stores a negative number. Hardware uses two's complement, so a negative value's bit pattern looks nothing like its positive counterpart.

Frequently Asked Questions

Can I convert many decimal numbers at once?

How do you convert decimal to octal?

Why does my file mode show as 33261?

Why is 0755 dangerous in code?

Conversion runs in your browser with exact integer arithmetic. Nothing is uploaded.