Octal to Decimal Converter – Free Online Tool

Convert Any Octal Number to Decimal Instantly

Type an octal number (base 8) above and get the decimal (base 10) result right away — no signup, no waiting, no uploading anything. The conversion happens entirely in your browser, so your data stays private on your device.

The tool also shows you the same value in binary and hexadecimal at the same time, which saves a lot of tab-switching when you're working across number systems.

How to Use the Octal to Decimal Converter

1. Type your octal number into the input box above. Valid digits are 0 through 7 only — no 8s or 9s.
2. The decimal result appears instantly as you type. No button to press.
3. Copy the decimal output with one click to paste wherever you need it.
4. Need the value in binary or hex too? Those fields update automatically alongside the decimal.

Worked Example: Octal 157 to Decimal

Let's convert the octal number 157 to decimal, step by step.

Each digit is multiplied by 8 raised to the power of its position, counting from the right starting at 0:

Octal: 1  5  7
Place: 8² 8¹ 8⁰

1 × 64 = 64
5 × 8  = 40
7 × 1  =  7
─────────────
Total  = 111 (decimal)

So octal 157 = decimal 111. The tool gives you this result the moment you type the last digit.

Quick Reference: Octal to Decimal Chart

Notice how octal 10 equals decimal 8 — that's the octal carry, the same way decimal 10 means "one ten and zero ones." This trips people up the first time.

How to Convert Octal to Decimal in Code

If you need to do this conversion inside a script rather than by hand, here are the one-liners you'll actually use.

Python

# Pass a string with the octal digits; prefix '0o' tells Python the base
result = int('157', 8)
print(result)  # 111

Python's built-in int() function accepts any base from 2 to 36 as its second argument, so this works for large numbers too.

JavaScript

// parseInt with radix 8 reads octal strings
const result = parseInt('157', 8);
console.log(result); // 111

The second argument — the radix — tells parseInt which base to read the input as. Always pass it explicitly; omitting it causes surprises with strings that start with zero.

How It Works Under the Hood

Octal is base 8, meaning each digit position represents a power of 8 (1, 8, 64, 512, …). Decimal is base 10, where each position is a power of 10. Converting between them is a matter of multiplying each octal digit by its place value and summing the results — exactly what the step-by-step example above shows.

The converter here uses JavaScript's native BigInt for very large numbers, so it won't lose precision the way a plain Number would for values beyond 2⁵³. The ECMAScript specification defines how these numeric types work in browsers.

When to Use Octal — and When to Skip It

Use octal when:

• Reading or writing Unix file permissions — chmod 755 and chmod 644 are octal values.
• Working with legacy systems or older C code that represents byte-level values in base 8.
• Learning how positional numeral systems work, since octal sits neatly between binary and hex.

Octal probably isn't what you need when:

• You're converting colors or memory addresses — hexadecimal is the standard there.
• You're working with network protocols or cryptography, where binary or hex is far more common.
• Your input contains digits 8 or 9 — those aren't valid octal and the tool will flag the error.

Privacy — Your Numbers Stay in Your Browser

Nothing you type is sent to a server. The entire octal to decimal calculation runs locally in your browser using JavaScript. Close the tab and it's gone — no logs, no storage, no account needed.

The tool is completely free to use as often as you like.

Need to work with data formats too? Check out our JSON Beautifier for readable, formatted JSON, or the JSON Validator to catch syntax errors fast. If you're switching between formats, the JSON to YAML Converter handles that in one step.

Ready? Type your octal value into the tool above and the decimal equivalent appears immediately.