C Program to Convert Octal Number to Decimal Number System

• Write a C program to convert octal number to decimal number system.
• How to convert octal number to binary number.

Required Knowledge

• C printf and scanf functions
• While loop in C

Octal number system is a base 8 number system using digits 0 and 7 whereas Decimal number system is base 10 number system and using digits from 0 to 9. Given an octal number as input from user convert it to decimal number.

For Example

2015 in Octal is equivalent to 1037 in Decimal number system.

Algorithm to convert Octal to Decimal number

• We multiply each octal digit with 8ⁱ and add them, where i is the position of the octal digit(starting from 0) from right side. Least significant digit is at position 0.

Let’s convert 2015(octal number) to decimal number
Decimal number = 2*8³ + 0*8² + 1*8¹ + 5*8⁰ = 1024 + 0 + 8 + 5 = 1037

C program to convert a octal number to decimal number

#include <stdio.h>  
#include <math.h>    
   
int main() {  
    long octalNumber, decimalNumber=0;  
    int position=0, digit;  
   
    printf("Enter an Octal Number\n");  
    scanf("%ld", &octalNumber);  
     
    /* Converting octal number to decimal number */
    while(octalNumber!=0) {   
        /* get the least significant digit of octal number */
 
        digit = octalNumber%10;
        decimalNumber += digit*pow(8, position);    
   
        position++;  
        octalNumber /= 10;  
    }  
  
    printf("Decimal Number : %ld", decimalNumber);  
   
    return 0;  
}

Output

Enter an Octal Number
2015
Decimal Number : 1037

Enter an Octal Number
1234
Decimal Number : 668