Musical Note Length Calculator in C This is my first ever program in C and I was just tinkering around with this idea, as I'm on a music based course and the synth plugins all use milliseconds as its time. 

Any improvements to make it more efficient or less dependant with a little explanation would be really appreciated.

/* declarations */
int crotchet(int);
int semibreve(int);
int minim(int);
int quaver(int);
int semiquaver(int);
int demisemiquaver(int);

int main() {

int bpm;

/* Ask for note bpm */
printf("Enter bpm value:\n");
scanf("%d", &amp;bpm);

/* returns note lengths from functions */
printf("Semibreve would last for... ");
printf("%d", semibreve(bpm));
printf(" miliseconds! \n");

return 0;
}

int crotchet(int bpm)
{
/*  1 second = 1000
 1 min = 60 seconds = 6000 miliseconds
 */

int crotchet;
crotchet = 6000 / bpm;

return crotchet;
}

int semibreve(int bpm)
{
int crot;
crot = crotchet(bpm);
int semibreve;
semibreve = crot * 4;

return semibreve;
}

int minim(int bpm)
{
int crot;
crot = crotchet(bpm);
int minim;
minim = crot * 2;

return minim;
}


Same for quaver (dividing crot by 2), semiquaver (dividing crot by 4) and demi semiquaver (dividing crot by 8).

Any additional explanation towards the music theory I will also add if requested.
Your calculations are all off by a factor of 10: one minute is 60000 milliseconds, not 6000.  Instead of explaining your 6000 using a comment, write it out in the code, and the compiler will figure it out for you. That calculation will not cost you any runtime performance at all.

I don't recommend that design, defining one function per type of note.  One function should cover everything, I think.

Since you use printf() and scanf(), you should #include &lt;stdio.h&gt;.

It is customary to put main() at the end, so that you don't have to declare the functions it uses in advance.

#include &lt;stdio.h&gt;

#define SEMIBREVE       (1.0)
#define MINIM           (1.0/2)
#define CROTCHET        (1.0/4)
#define QUAVER          (1.0/8)
#define SEMIQUAVER      (1.0/16)
#define DEMISEMIQUAVER  (1.0/32)

int millisec(int bpm, double note) {
    return (int)(
        60      /* seconds */
        * 1000  /* milliseconds per second */
        * 4     /* crotchets per semibreve */
        * note
        / bpm
    );  
}    

int main() {
    int bpm;
    printf("Enter tempo (crotchets per minute): ");
    scanf("%d", &amp;bpm);

    printf("Semibreve would last for... %d milliseconds!\n", millisec(bpm, SEMIBREVE));
    printf("Dotted crotchet would last for... %d milliseconds!\n", millisec(bpm, 1.5 * CROTCHET));
    printf("Demisemiquaver would last for... %d milliseconds!\n", millisec(bpm, DEMISEMIQUAVER));
}