About octal and decimal

Octal is base 8, using digits 0–7. Each octal digit maps to exactly three bits, which is why Unix file permissions like 755 are octal. Code writes it with a 0o prefix.

Decimal is base 10 — ordinary numbers. Most code displays values in decimal even when they're stored as bits underneath, so it's usually one side of any base conversion.

For example, octal 755 is 493 in decimal. The converter uses BigInt, so numbers of any size convert without losing precision — and it all runs in your browser.