The octal numeral system, or oct for short, is the base-8 number system, and uses the digits 0 to 7, that is to say 10 represents 8 in decimal and 100 represents 64 in decimal. However, English uses a base-10 number language system and so a true octal system might use different language to avoid confusion with the decimal system.
In the decimal system, each place is a power of ten. For example:
74 10 = 7 × 10 1 + 4 × 10 0 {\displaystyle \mathbf {74} _{10}=\mathbf {7} \times 10^{1}+\mathbf {4} \times 10^{0}}In the octal system, each place is a power of eight. For example:
112 8 = 1 × 8 2 + 1 × 8 1 + 2 × 8 0 {\displaystyle \mathbf {112} _{8}=\mathbf {1} \times 8^{2}+\mathbf {1} \times 8^{1}+\mathbf {2} \times 8^{0}}By performing the calculation above in the familiar decimal system we see why 112 in octal is equal to 64+8+2 = 74 in decimal.
Octal numerals can be easily converted from binary representations (similar to a quaternary numeral system) by grouping consecutive binary digits into groups of three (starting from the right, for integers). For example, the binary representation for decimal 74 is 1001010. Two zeroes can be added at the left: (00)1 001 010, corresponding the octal digits 1 1 2, yielding the octal representation 112.
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