diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..2f8d3ac
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,2 @@
+build:
+	gcc -Wall -pedantic hexcalc.c
diff --git a/hexcalc.c b/hexcalc.c
new file mode 100644
index 0000000..480ccb8
--- /dev/null
+++ b/hexcalc.c
@@ -0,0 +1,260 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <ctype.h>
+
+/* THIS IS A WORK IN PROGRESS.
+ *
+ * This is my first program in C and I look forward to learning more and making
+ * those changes as I go along. Feedback is appreciated, for example -Wall and
+ * -pedantic (see Makefile) were very appreciated.
+ *
+ * I'm not looking for perfection.
+ *
+ * The goal is to later maybe putting this into an ATTiny or Atmega328 and put
+ * it into some hardware. Just for fun with a display and inputs and stuff.
+ *
+ * What I learned writing this:
+ * - Internet searches return pretty unhelpful stuff
+ * - ... but I found out that there is something called ispunct, isupp, isalnum
+ *   and a lot of other helpful functions that are part of the C standard
+ *   library. If the validation thing even is something I want.
+ *   - but trying to find out what I wanted about these were harder,
+ *   then I remember I have manpages installed - but how do I find them? man
+ *   stdlib.h, stdio.h and string.h didn't have anything, but man ispunct lead
+ *   me to man ctype.h where I found everything I needed.
+ *
+ * TODO/Need to look into:
+ * - Freeing memory after use
+ * - Use valgrind
+ * - Look for null pointers and stuff
+ * - Read about pointers
+ * - Padding for binary number output
+ * - Padding for decimal output
+ * - Decimal to hex converter
+ * - Other things mentioned in the code
+*/
+
+#define LINE 20
+
+// declare functions - guess these should go into a header file?
+// the error messages I got before putting this here were not helpful.
+char* validate_input(char*);
+char* to_hex(char*);
+int hex_to_decimal(char*, char*, int);
+char* to_binary(unsigned int, char*);
+char* binary_string(char*, char*);
+int binary_to_decimal(char*, char*);
+
+char hex[4]; // 4 characters (ff ff)
+int  bin[8]; // 8 digits (1111 1111)
+int  dec[5]; // 5 digits (65 535)
+
+int main() {
+  char* buffer = malloc(LINE+1);
+  char buf2[17];
+  char buf3[LINE+1];
+  int i;
+  int ch;
+
+  printf("Enter a number and this program will convert the number to decimal, hexadecimal \n");
+  printf("and binary.\n");
+
+  while (1) {
+    printf("\n> ");
+
+    for (i = 0; (i < LINE) && (( ch = getchar()) != EOF) && (ch !='\n'); i++)
+      buffer[i] = ch;
+
+    buffer[i] = '\0';
+    int size = i;
+
+    if (buffer[0] == '\0')
+      break;
+
+    //printf("Given string was: '%c'\n", *test);
+
+    char *test = validate_input(buffer);
+    int in_decimal;
+    char *in_binary;
+
+    in_decimal = hex_to_decimal(buffer, buf2, size);
+    in_binary = to_binary(in_decimal, buf2);
+    printf("%s as hex:\n", buffer);
+    printf("decimal> %i\n", in_decimal);
+    printf("binary> %s\n", binary_string(in_binary, buf3));
+    printf("\n");
+
+    if (*test == 'd') {
+      printf("%s as decimal:\n", buffer);
+      in_binary = to_binary(atoi(buffer), buf2);
+      printf("hex> \n");
+      printf("binary> %s\n", binary_string(in_binary, buf3));
+    }
+
+    if (*test == 'b') {
+      printf("%s as binary:\n", buffer);
+      int binary_in_decimal = binary_to_decimal(buffer, buf2);
+      printf("hex> \n"); // TODO: Convert binary to hex
+      printf("decimal> %i\n", binary_in_decimal);
+    }
+
+    free(test);
+  }
+
+  printf("Exiting program.\n");
+
+  return 0;
+}
+
+char* binary_string(char* in_binary, char* buffer) {
+  int j = 0; // for padding
+  for (int i = 0; i < 16; i++) {
+    buffer[i] = (int) in_binary[i];
+  }
+  buffer[17] = '\0';
+
+  return buffer;
+}
+
+// TODO:
+//char* to_hex(char* input) {
+//}
+
+int hex_to_decimal(char* input, char *buff, int size) {
+  char *pbuff = buff;
+  int newbuff[8];
+  int newbuff_used;
+
+  char *ch = input;
+  ch += size;
+
+  *pbuff = 1;
+
+  // TODO: Shouldn't have to do this. Look into atoi()?
+  // 0=48
+  // a=97
+  for (int i = 0; i < LINE; i++) {
+    *ch--;
+    if (*ch == '\0') {
+      break;
+    }
+    if (*ch >= 97) {
+      int tmp = *ch - 87;
+      newbuff[i] = tmp;
+    } else {
+      newbuff[i] = *ch - 48;
+    }
+    newbuff_used = i+1;
+  }
+
+  int result = 0;
+  int exp;
+  for (int i = 0; i < newbuff_used; i++) {
+    exp = i;
+
+    while (exp > 0) {
+      newbuff[i] *= 16;
+      --exp;
+    }
+
+    result += newbuff[i];
+  }
+
+  return result;
+}
+
+char* to_binary(unsigned int input, char *buff) {
+  char *pbuff = buff;
+
+  int size = 16;
+
+  pbuff += size;
+  *pbuff-- = '\0';
+
+  for (int i = 0; i < size; i++) {
+    *pbuff-- = ((input & 1) == 1) ? '1' : '0';
+
+    input >>= 1;
+
+  }
+  return pbuff+1;
+}
+
+int binary_to_decimal(char* input, char *buff) {
+  char *pbuff = input;
+
+  int size = strlen(input);
+  printf("size: %i\n", size);
+  int result = 0;
+
+  pbuff += size;
+  int exp = 1;
+
+  for (int i = 0; i < size; i++) {
+    *pbuff--;
+
+    int val = (int) *pbuff;
+    val -= 48; // TODO: Shouldn't have to do this. Look into atoi()?
+
+    result += val * exp;
+
+    exp *= 2;
+  }
+
+  return result;
+}
+
+// returns b for binary, h for hex, d for decimal or e for error
+char* validate_input(char* input) {
+  int b_count    = 0; // binary
+  int h_count    = 0; // hex (hexadecimal)
+  int d_count    = 0; // decimal (alphanumerical)
+  int a_count    = 0; // letters (alpha)
+  int length     = 0;
+
+  int i;
+  char *ret = malloc(1);
+
+  for (i = 0; i < 8; i++) {
+    if (input[i] == '\0') {
+      printf("\n");
+      break;
+    }
+
+    if (input[i] == '0' || input[i] == '1')
+      b_count++;
+
+    if (isdigit(input[i]))
+      d_count++;
+
+    if (isalpha(input[i]))
+      a_count++;
+
+    if (isxdigit(input[i])) {
+      h_count++;
+    } else {
+      printf("%c error\n", input[i]);
+      ret[0] = 'e';
+      return ret;
+    }
+
+    length++;
+  }
+
+  // if it contains letters it's not binary and not a decimal
+  if (a_count == 0)
+    ret[0] = 'd';
+
+  // Hex is written in groups of 2, like ff or 0afe, not fad.
+  if (a_count != 0 && length == h_count && (h_count == 2 || h_count == 4))
+    ret[0] = 'h';
+
+  // Binary is written in groups of 2 (or 4), like 10, 11, 1011 and 0100, not 1 or 101.
+  if (a_count == 0 && length == b_count && (b_count % 2 == 0))
+    ret[0] = 'b';
+
+  //printf("%-1c: %d d, %d h, %d b, %d length\n\n", ret[0], d_count, h_count, b_count, length);
+
+  return ret;
+}