Octal

Octal is a counting system that uses eight digits.

Instead of using only 0's and 1's like binary, or the characters '0' to '9' of the decimal number system; octal uses the characters '0' to '7', so does not need what would normally be classed as alphabetic characters to represent digits as Hexadecimal does.

Uses

The octal number system was used widely in the Electronics and Computer Industry, as although digital electronics is based on gates with only two states and is therefore fundamentally binary, binary numbers can quickly become long and hard to transcribe without errors. Their octal equivalents are much shorter and easier to remember, and have a straight-forward way of conversion to/from binary.

The PDP-11 computer made by the Digital Equipment Corporation used the octal numeric system exclusively for displaying memory addresses and content.

Unix file system permissions have three sets (user, group, others) of three bit permissions (read, write, execute), which is naturally represented in octal.

The use of octal numbers has declined as most modern computers no longer base their word length on multiples of three bits, (they are based on multiples of four bits, so hexadecimal is more widely used).

Comparing counts from zero in different number systems

C.f. Common number base formatting and Common number base parsing

     Binary
         Octal
            Decimal
     0   0  0
     1   1  1
    10   2  2
    11   3  3
   100   4  4
   101   5  5
   110   6  6
   111   7  7
  1000  10  8
  1001  11  9
  1010  12 10
  1011  13 11
  1100  14 12
  1101  15 13
  1110  16 14
  1111  17 15
 10000  20 16
 10001  21 17
 10010  22 18
 10011  23 19
 10100  24 20
 10101  25 21

Converting binary to octal

1. Split a binary number into groups of three digits, counting from right to left.
2. Pad the leftmost group of binary digits with zeros on their left if their are less than three digits.
3. Use the following table to translate each group of three binary digits, in order, to its octal equivalent.

Binary digits
     Octal equivalent digit
000  0
001  1
010  2
011  3
100  4
101  5
110  6
111  7

An example conversion

     Binary Number:     1011010111
             Split:  1 011 010 111
               Pad:001 011 010 111
  Translate groups:  1   3   2   7
      Octal answer:           1327