Convert Octal to Decimal

Convert octal (base 8) numbers to decimal instantly — free online converter.

About

About the Octal to Decimal Converter

Octal literals still show up in older code and in Unix permission codes like 755, and converting octal to decimal turns that base-8 notation back into the ordinary number most people find easier to reason about.

Octal place values are powers of 8 — 1, 8, 64, 512 — so 644 is 6×64 + 4×8 + 4 = 420. That difference catches people out in code: in C, and in older non-strict JavaScript, a number written with a leading zero like 0644 is read as octal, so it means 420, not six hundred and forty-four.

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Guide

Using the Octal to Decimal Converter

  1. 1

    Step 1

    Octal is already selected below as the input base.

  2. 2

    Step 2

    Type your octal digits (0-7) into the value box.

  3. 3

    Step 3

    The result table fills in instantly — the decimal value sits alongside binary, hex and octal all at once.

  4. 4

    Step 4

    For a quick manual check, multiply each octal digit by the matching power of 8 (counting from the right, starting at 8^0) and add the results.

  • Converts octal to decimal as you type
  • Flags 8 and 9 straight away as invalid octal digits
  • Accepts a 0o prefix or a leading zero, exactly as written in code
  • Shows the value in binary, octal, decimal, hexadecimal and base 36 at once, plus any custom base from 2 to 36
  • Exact for whole numbers up to 9,007,199,254,740,991, and tells you when a number is too large
  • Runs entirely in your browser — free, with no sign-up and no limit

Developers

Developers find out what a leading-zero literal really equals before it causes a bug.

Linux administrators

Admins compare a chmod mode with the decimal mode number some tools and APIs print.

Programmers decoding strings

Programmers turn an octal escape like \101 into its character code, 65.

Students

Students check powers-of-8 conversions.

Popular base conversions

Binary Octal Binary Decimal Binary Hexadecimal Octal Binary Octal Hexadecimal Decimal Binary Decimal Octal Decimal Hexadecimal Hexadecimal Binary Hexadecimal Octal Hexadecimal Decimal

Examples

Octal to Decimal examples

4 worked examples

01

chmod 755 permission

Octal 755 converts to decimal 493, calculated as (7×64) + (5×8) + (5×1).

02

A small octal value

Octal 17 converts to decimal 15, calculated as (1×8) + (7×1).

03

A round octal number

Octal 1000 converts to decimal 512, since 8³ = 512.

04

File permissions 644

644 converts to 420 in decimal.

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FAQ

Frequently asked questions

Quick answers to common questions about the Octal to Decimal Converter.

How do I convert octal to decimal?

Type your octal value above, with octal selected as the input base. The decimal value appears instantly in the results table — nothing is uploaded anywhere.

How do you convert octal to decimal by hand?

Multiply each digit by 8 raised to its position (from the right, starting at 0) and add them up: 755 is (7×8²)+(5×8¹)+(5×8⁰) = 448+40+5 = 493.

What is chmod 755 in decimal?

755 in octal is 493 in decimal — the same permission value the operating system stores internally behind the more readable octal chmod notation.

Is this octal to decimal converter free?

Yes. It's free, with no sign-up and no limit on how many numbers you convert.

Can I convert decimal back to octal?

Yes — the same converter works in both directions. Select decimal as the input base instead, or use the dedicated decimal to octal converter page.

How large an octal value can I convert?

Up to 9,007,199,254,740,991 in decimal terms — beyond that the conversion won't be exact, and the tool will tell you so.

Can I paste an octal value with a 0o or 0 prefix?

Yes. 0o is removed automatically and a leading 0 doesn't change the value, so 0o17, 017 and 17 all give 15.

What happens if I type an 8 or 9?

The input is rejected with a message listing the allowed digits, 0 to 7, rather than being converted to a wrong value.

What is chmod 644 in decimal?

420. Each octal digit is worth eight times the one to its right, so 6 × 64 + 4 × 8 + 4 = 420.

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