Octal to Decimal Converter — Base 8 to Base 10 Online

Octal

Decimal

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How to use Octal to Decimal

Enter the octal number — digits 0–7 only, e.g. 755.
Read the decimal instantly: 755 → 493.
Check the error line if a token contains 8 or 9 — those digits don't exist in base 8 and are flagged by name.
Click “Copy result” to use the decimal value anywhere.

What is Octal to Decimal?

An octal to decimal converter reads a base-8 number and returns its base-10 value. Octal places are worth 1, 8, 64, 512… from the right, so 755 in octal is 7×64 + 5×8 + 5 = 493 in decimal — a different number than it looks, which is exactly why the conversion trips people up.

That deceptive familiarity is octal's hazard: every octal number looks like a valid decimal number. 100 in octal is 64 in decimal; 17 is 15. Several programming languages read a leading zero as octal (010 = 8, not 10), a design decision that has produced decades of subtle bugs.

About the Octal to Decimal

Type an octal value and its decimal equivalent appears instantly; multiple space-separated values convert independently, with 8s and 9s flagged as invalid rather than misread.

Uses: converting a Unix permission like 644 to its numeric value 420 when an API or config wants plain decimal, decoding octal escape sequences in strings (\101 is 65, the letter A), working through number-systems homework, and untangling legacy code where a leading zero silently made a constant octal. BigInt math keeps long values exact.

Runs live in your browser. Decimal to Octal reverses the direction; Octal to Binary shows the same value as permission-style bit triplets.

Frequently Asked Questions

Multiply digits by place values 1, 8, 64, 512… from the right and add: 327 = 3×64 + 2×8 + 7 = 192 + 16 + 7 = 215.

493 — but note that chmod 755 is used as three separate digits (7, 5, 5 = rwx, r-x, r-x), so the combined decimal value rarely matters for permissions; it matters when a config or API stores the mode as one plain number.

C, Java, older JavaScript and others treat a leading zero as an octal prefix: 010 parses as base 8 = 8. Modern style uses the explicit 0o10 prefix to avoid exactly this surprise.

A character code written in octal inside a string: \101 is octal 101 = decimal 65 = the letter A. C and many Unix tools still accept these.

Only from context or prefix — the digits alone can't tell you. In octal, 100 = 64. That ambiguity is why explicit prefixes (0o100) exist and why this tool asks you, not the number, which base you mean.

Learn more

Octal Numbers Explained (Base 8)

Octal shows up in file permissions and older systems. Here is what base 8 is and how to convert it.

Read the guide