01 #include <stdio.h>
02 #include <math.h>
03 #include <string.h>
04 
05 int main()
06 {
07     char hex[17];
08 
09     long int decimal=0;
10 
11     int i = 0, val, len;
12 
13     printf("Enter any hexadecimal number: ");
14     gets(hex);
15 
16     /* length of hex digit */
17 
18     len = strlen(hex);
19     len--;
20 
21     for(i=0; hex[i]!=''; i++)
22         {
23             switch(hex[i])
24                 {
25                 case '0':
26                     val = 0;
27                     break;
28                 case '1':
29                     val = 1;
30                     break;
31                 case '2':
32                     val = 2;
33                     break;
34                 case '3':
35                     val = 3;
36                     break;
37                 case '4':
38                     val = 4;
39                     break;
40                 case '5':
41                     val = 5;
42                     break;
43                 case '6':
44                     val = 6;
45                     break;
46                 case '7':
47                     val = 7;
48                     break;
49                 case '8':
50                     val = 8;
51                     break;
52                 case '9':
53                     val = 9;
54                     break;
55                 case 'a':
56                 case 'A':
57                     val = 10;
58                     break;
59                 case 'b':
60                 case 'B':
61                     val = 11;
62                     break;
63                 case 'c':
64                 case 'C':
65                     val = 12;
66                     break;
67                 case 'd':
68                 case 'D':
69                     val = 13;
70                     break;
71                 case 'e':
72                 case 'E':
73                     val = 14;
74                     break;
75                 case 'f':
76                 case 'F':
77                     val = 15;
78                     break;
79                 }
80 
81             decimal += val * pow(16, len);
82             len--;
83         }
84 
85     printf("Decimal number = %ld",decimal);
86     return 0;
87 }

 OUTPUT :
 
 Enter any hexadecimal number : 1E
 Decimal number = 30