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Java Program to Convert Octal to Decimal

Last Updated : 23 Jul, 2025

Octal numbers are numbers with 8 bases and use digits from 0-7. This system is a base 8 number system. The decimal numbers are the numbers with 10 as their base and use digits from 0-9 to represent the decimal number. They also require dots to represent decimal fractions.

We have to convert a number that is in the Octal Number System to the Decimal Number System. The base in an Octal Number is 8, which means that an Octal Number will have digits ranging from 0 to 7.

Basic Input/Output to Convert Octal To Decimal:

Input: 167 (Octal)
Output: (7 * 8⁰) + (6 * 8¹) +(1 * 8²)=119 (Decimal)

The diagram below explains how to convert an octal number (123) to an equivalent decimal value:

There are two main methods to convert Octal to Decimal in Java. They are:

• Using Integer.parseInt() Method
• Custom Method

To convert any string form to decimal, we can use type.parseType() method. For example, here we need to convert from octal to decimal, and the octal form is an integer, so we can use Integer.parseInt() to convert it.

Example: Java program for octal to decimal number conversion using Integer.parseInt() method.

// Java program to convert
// octal number to decimal using
// Integer.parseInt()
public class Geeks
{
    public static void main(String args[])
    {
        // octal value
        String oct = "157";

        // octal to decimal using Integer.parseInt()
        int num = Integer.parseInt(oct, 8);

        System.out.println(
            "Decimal equivalent of Octal value of 157 is: "
            + num);
    }
}

Decimal equivalent of Octal value of 157 is: 111

Complexity Analysis:

• Time complexity : O(1)
• Auxiliary space : O(1)

Algorithm:

• Start and take the octal input from the user.
• Create a result variable with an initial value of 0 to store the resultant Decimal number.
• Create a loop for getting every digit in the Input.
• Multiply each digit in the number with 8^n-1, where n is the digit's position.
• Then add it to the result.
• Store the value in Step(5) to the result variable.
• Print the result variable.

Example: Java program for octal to decimal number conversion using custom method.

// Java program to convert octal
// to decimal number using custom code
import java.lang.Math;

// Driver Class
public class Geeks
{
    // main function
    public static void main(String[] args)
    {
        int a = 167;

        // Initialize result variable to 0.
        int res = 0;

        // Take a copy of input
        int copy_a = a;

        for (int i = 0; copy_a > 0; i++) {

            // Take the last digit
            int temp = copy_a % 10;

            // Appropriate power on 8 suitable to
            // its position.
            double p = Math.pow(8, i);

            // Multiply the digits to the  into the Input
            // and
            //  then add it to result
            res += (temp * p);
            copy_a = copy_a / 10;
        }

        System.out.print("Decimal of Octal Number (" + a
                         + ") : " + res);
    }
}

Decimal of Octal Number (167) : 119

Complexity Analysis:

• Time complexity: O(logN) for given octal number N.
• Auxiliary space : O(1)

Please refer to the article Program for Octal to Decimal Conversion for more information.

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