How to convert from octal to decimal?

Assume we have a n-digits octal number:

d_n-1 ... d₃ d₂ d₁ d₀

Multiply each digit of the octal number with its corresponding 8ⁿ, where n is the position of the digit.

decimal number = d₀×8⁰ + d₁×8¹ + d₂×8² + ...

Convert the fractional part from octal to decimal:

Multiply each digit of the fractional digit with its corresponding 8^-n, where n is the position of the digit.

0.d₀ d₁ d₂ ... d_n-1

fractional part = d₀×8^-1 + d₁×8^-2 + ... + d_n-1×8ⁿ

Example:

123₈ = (1 × 8²) + (2 × 8¹) + (3 × 8⁰) + (4 × 8^-1) + (5 × 8^-2) + (6 × 8^-3) = 83.58984375₁₀

Below is the reference table for converting binary number to decimal, octal, hexadecimal, ranging from 0₁₀ to 15₁₀: