C Programming Code Examples C > For Loops and While Loops Code Examples program to convert Hexadecimal to Octal number system program to convert Hexadecimal to Octal number system Write a C program to input Hexadecimal number from user and convert it to Octal number system. How to convert Hexadecimal number system to Octal number system in C programming. Hexadecimal number system Hexadecimal number system is a base 16 number system. It uses 16 symbols to represent all numbers i.e. 0123456789ABCDEF Octal number system Octal number system is a base 8 number system. It uses 8 symbols to represent all numbers i.e. 01234567 There isn't any easy and direct conversion algorithm from hexadecimal to octal number system. Below is the step by step descriptive logic to convert hexadecimal to decimal number system. Convert the given hex number to binary. Group the converted binary in groups of 3 bits. Write the corresponding octal value of for each grouped binary number digit. Binary to Octal conversion table Decimal Binary Octal 0 000 0 1 001 1 2 010 2 3 011 3 4 100 4 5 101 5 6 110 6 7 111 7 #include <stdio.h> int main() { char hex[17]; long long octal, bin, place; int j = 0, rem, val; /* Input hexadecimal number from user */ printf("Enter any hexadecimal number: "); gets(hex); octal = 0ll; bin = 0ll; place = 0ll; /* Hexadecimal to binary conversion */ for(j=0; hex[j]!='\0'; j++) { bin = bin * place; switch(hex[j]) { case '0': bin += 0; break; case '1': bin += 1; break; case '2': bin += 10; break; case '3': bin += 11; break; case '4': bin += 100; break; case '5': bin += 101; break; case '6': bin += 110; break; case '7': bin += 111; break; case '8': bin += 1000; break; case '9': bin += 1001; break; case 'a': case 'A': bin += 1010; break; case 'b': case 'B': bin += 1011; break; case 'c': case 'C': bin += 1100; break; case 'd': case 'D': bin += 1101; break; case 'e': case 'E': bin += 1110; break; case 'f': case 'F': bin += 1111; break; default: printf("Invalid hexadecimal input."); } place = 10000; } place = 1; /* Binary to octal conversion */ while(bin > 0) { rem = bin % 1000; switch(rem) { case 0: val = 0; break; case 1: val = 1; break; case 10: val = 2; break; case 11: val = 3; break; case 100: val = 4; break; case 101: val = 5; break; case 110: val = 6; break; case 111: val = 7; break; } octal = (val * place) + octal; bin /= 1000; place *= 10; } printf("Hexadecimal number = %s\n", hex); printf("Octal number = %lld", octal); return 0; }