The octal numeral system is a base 8 numeral system. It uses the numerals 0 through 7. The system is similar to binary (base 2) and hexadecimal (base 16). Octal numerals are written using the letter o before the numeral, for example, o04 or o1242. Octal numbers are also sometimes written with a small 8 to the lower right, as in 1242_{8}.
At one time, the octal system was used mainly for work with computers. It provided an easier way to work with binary numbers. As computers changed from using 24bit systems to 32 and 64bit systems, hexadecimal replaced octal for most uses. Certain groups, for example, Native Americans using the Yuki language in California and the Pamean languages in Mexico, also use an octal numbering system. They do this because when they count, they use the spaces between their fingers instead of counting the actual fingers.
Octal and binary
The octal numeral system uses a "threebit" binary coding. Each digit in an octal numeral is the same as three digits in a binary numeral. The grouping of the binary digits is done from right to left. The first three binary digits from the right are grouped into the last part of the octal numeral, then the next three digits form the next to the last part of the numeral.
Octal 
Binary 

1 
001 
2 
010 
3 
011 
4 
100 
5 
101 
6 
110 
7 
111 
10 
001 000 

Octal 
Binary 

11 
001 001 
12 
001 010 
36 
011 110 
45 
100 101 
53 
101 011 
64 
110 100 
100 
001 000 000 
357 
011 101 111 

Binary 
Groupings 
Octal 

11 



011 
3 
010111 


010 
111 
27 
101000110 

101 
000 
110 
506 
01011010101 
001 
011 
010 
101 
1325 

Octal and decimal
In the decimal system (base 10), each digit in octal is equal to that digit multiplied by the exponent of 8 that is equal to its location minus one.

Location 

6 
5 
4 
3 
2 
1 
Value 
32768 (8^{5}) 
4096 (8^{4}) 
512 (8^{3}) 
64 (8^{2}) 
8(8^{1}) 
1 (8^{0}) 

Example: o3425 to decimal

Octal 

Decimal 

o3425 
= 
( 5 × 1 ) 
+ 
( 2 × 8) 
+ 
( 4 × 64 ) 
+ 
( 3 × 512) 
= 
5 
+ 
16 
+ 
256 
+ 
1536 
o3425 
= 
1813 

Octal and hexadecimal
Octal is similar to hexadecimal because they are both easily converted to binary. Where octal is equal to threedigit binary, hexadecimal is equal to fourdigit binary. Where octal numerals start with the letter "o", hexadecimal numerals end with the letter "h". The easiest way to convert from one to the other is to convert to binary and then to the other system.
Octal 
Binary 
Hexadecimal 

three digit 
four digit 
o4 




100 



0100 
04h 
o15 



001 
101 



1101 
0Dh 
o306 


011 
000 
110 


1100 
0110 
C6h 
o54253 
101 
100 
010 
101 
011 
0101 
1000 
1010 
1011 
58ABh 
Related pages
See also
In Spanish: Sistema octal para niños