Read the ambient air pressure instruments on the dash of your aircraft. Note the number. For example, your instrument might read "165".
Multiply each digit in the number by 8 to the power of the number of digits that digit lies to the left, starting with zero. For example, if the number is 165, the first digit is 5, which should be multiplied by 8 to the power of 0 because it's zero digits to the left.
5 x 8 ^ 0
5 x 1
5
The next digit is 6:
6 x 8 ^ 1
6 x 8
48
Then:
1 x 8 ^ 2
1 x 64
64
Add all the numbers from the previous step together. For example, adding the numbers from the example above gives you 5 + 48 + 64 = 117. Thus, 117 is the decimal representation of octal 165.