bsample.h
=========
#include <stdio.h>
#define	AREA				2337
#define	SQ_MILES_PER_SQ_KILOMETER	0.3861021585424458
#define	SQ_FEET_PER_SQ_MILE		(5280 * 5280)
#define	SQ_INCHES_PER_SQ_FOOT		144
#define	ACRES_PER_SQ_MILE		640

bsample.c
=========
/* Measuring the Pacific sea */
#include "bsample.h"
main()
{
  const int	pacific_sea = AREA; /* in sq kilometers */
  double	acres, sq_miles, sq_feet, sq_inches;

  printf("\nThe Pacific sea covers an area");
  printf(" of %d square kilometers.\n", pacific_sea);
  sq_miles = SQ_MILES_PER_SQ_KILOMETER * pacific_sea;
  sq_feet = SQ_FEET_PER_SQ_MILE * sq_miles;
  sq_inches = SQ_INCHES_PER_SQ_FOOT * sq_feet;
  acres = ACRES_PER_SQ_MILE * sq_miles;
  printf("In other units of measure this is:\n\n");
  printf("%22.7e acres\n", acres);
  printf("%22.7e square miles\n", sq_miles);
  printf("%22.7e square feet\n", sq_feet);
  printf("%22.7e square inches\n", sq_inches);
}

Output:

The Pacific sea covers an area of 2337 square kilometers.
In other units of measure this is:

         5.7748528e+05 acres
         9.0232074e+02 square miles
         2.5155259e+10 square feet
         3.6223572e+12 square inches