Octal Numbers Explained (Base 8)

Octal (base 8) is less common than binary or hex today, but it still appears in important places — most famously Unix file permissions. Here's the gist.

What base 8 means

Octal uses digits 0–7, with each place a power of 8 (1, 8, 64…). So octal 17 = (1×8) + 7 = 15 in decimal. Each octal digit maps neatly to exactly 3 binary bits, which is why it pairs well with binary.

Where you'll meet it

The classic use is Unix/Linux file permissions — chmod 755 is octal, where each digit (owner, group, others) encodes read/write/execute as a sum (4+2+1). That's octal in everyday action.

Converting

Our free converters handle it: octal to decimal, decimal to octal, octal to binary and octal to hex. For the bigger picture, see our number systems guide.

Bottom line

Octal is base 8 (digits 0–7), each digit = 3 bits, and its headline modern use is Unix file permissions like chmod 755. Niche, but worth recognising when you see it.