can someone help me fine tune this code a little more!? this code converts units and uses pass by ref (which it must) but I need help refining it! how can I improve this code!?
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#include <iostream>
#include <math.h>
using namespace std;

enum ConversionType {LENGTH_TO_METRIC, LENGTH_TO_US, WEIGHT_TO_METRIC, WEIGHT_TO_US};
void convert_lengths();
void convert_weights();
void length_to_metric(ConversionType conversion);
void length_to_us(ConversionType conversion);
void weight_to_metric(ConversionType conversion);
void weight_to_us(ConversionType conversion);
void prompt_user(int& large_unit, double& small_unit, ConversionType conversion);
void convert(int large_unit, double small_unit, int& new_large, double& new_small, ConversionType conversion);
void output_results(int large_before, double small_before, int large_after, double small_after, ConversionType conversion);

int main()
{
       int value;
       do {
              do {
                     cout <<"Would you like to convert the weight or length:\n"
                              "Press 1 for Lengths\n"
                              "Press 2 for Weight or\n"
                              "Press 0 to terminate program\n";
                     cin >> value;
                     if (value != 0 && value != 1 && value != 2)
                           cout << "Invalid entry." << endl;
                     } while (value != 0 && value != 1 && value != 2); 
              switch (value)
              {
                     case 0: 
                           break;
                     case 1: convert_lengths(); 
                           break;
                     case 2: convert_weights();
                           break;
              } 
       } while(value != 0);

       return 0;
}

void convert_lengths()
{
       cout << "We are going to convert lengths for you." << endl;
       int value;
       do 
       {
              cout << "Would you like to convert:\n"
                           "Feet/Inches to Meters/Centimeters? If so press 1.\n"
                           "Meters/Centimeters to Feet/Inches? If so press 2. \n"
                           "Press 0 to quit.\n";
              cin >> value;
              if (value !=0 &&value !=1 && value !=2)
                     cout << "Invalad entry." << endl;
       } while (value !=0 &&value !=1 && value !=2);
       switch (value)
       {
              case 0: break;
              case 1: length_to_metric(LENGTH_TO_METRIC);     break;
              case 2: length_to_us(LENGTH_TO_US);      break;
       }
}

void convert_weights()
{
       cout << "We are going to convert weights for you." << endl;
       int value;
       do 
       {
              cout << "Would you like to convert:\n"
                           "Pounds/ounces to Kilograms/grams? If so press 1.\n"
                           "Kilograms/Grams to pounds/ounces? If so press 2.\n"
                           "Press 0 to quit.\n";
              cin >> value;
              if (value !=0 &&value !=1 && value !=2)
                     cout << "Invalad entry." << endl;
       } while (value !=0 &&value !=1 && value !=2);
       switch (value)
       {
              case 0: break;
              case 1: weight_to_metric(WEIGHT_TO_METRIC);     break;
              case 2: weight_to_us(WEIGHT_TO_US);      break;
       }
}

void length_to_metric(ConversionType conversion)
{
       int feet, meters;
       double inches, centimeters;
       cout << "You have chosen to convert Feet/Inches to Meters/Centimeters.\n";
       prompt_user(feet, inches, LENGTH_TO_METRIC);
       convert(feet, inches, meters, centimeters, LENGTH_TO_METRIC);
       output_results(feet, inches, meters, centimeters, LENGTH_TO_METRIC);

}

void length_to_us(ConversionType conversion)
{
       int meters, feet;
       double centimeters, inches;
       cout << "You have chosen to convert Meters/Centimeters to Feet/Inches.\n";
       prompt_user(meters, centimeters, LENGTH_TO_US);
       convert(meters, centimeters, feet, inches, LENGTH_TO_US);
       output_results(meters, centimeters, feet, inches, LENGTH_TO_US);
}

void weight_to_metric(ConversionType conversion)
{
       int pounds, kilograms;
       double ounces, grams;
       cout << "You have chosen to convert Pounds/Ounces to Kilograms/Grams.\n";
       prompt_user(pounds, ounces, WEIGHT_TO_METRIC);
       convert(pounds, ounces, kilograms, grams, WEIGHT_TO_METRIC);
       output_results(pounds, ounces, kilograms, grams, WEIGHT_TO_METRIC);
}

void weight_to_us(ConversionType conversion) 
{
       int kilograms, pounds;
       double grams, ounces;
       cout << "You have chosen to convert Kilograms/Grams to Pounds/Ounces.\n";
       prompt_user(kilograms, grams, WEIGHT_TO_US);
       convert(kilograms, grams, pounds, ounces, WEIGHT_TO_US);
       output_results(kilograms, grams, pounds, ounces, WEIGHT_TO_US);
}

void prompt_user(int& bigger_unit, double& smaller_unit, ConversionType conversion) 
{

       switch (conversion) {
       case LENGTH_TO_METRIC:
              cout << "Please enter the length:\n"
                           "Feet(integer): ";
              cin >> static_cast<int>(bigger_unit);
              cin.sync();
              cout << "Inches(decimal): ";
              cin >> static_cast<double>(smaller_unit);
              cin.sync();
              break;
       case LENGTH_TO_US:
              cout << "Please enter the length:\n"
                           "Meters(integer): ";
              cin >> static_cast<int>(bigger_unit);
              cin.sync();
              cout << "Centimeters(decimal): ";
              cin >> static_cast<double>(smaller_unit);
              cin.sync();
              break;
       case WEIGHT_TO_METRIC:
              cout << "Please enter the weight:\n"
                           "Pounds(integer): ";
              cin >> static_cast<int>(bigger_unit);
              cin.sync();
              cout << "Ounces(decimal): ";
              cin >> static_cast<double>(smaller_unit);
              cin.sync();
              break;
       case WEIGHT_TO_US:
              cout << "Please enter the weight:\n"
                           "Kilograms(integer): ";
              cin >> static_cast<int>(bigger_unit);
              cin.sync();
              cout << "Grams(decimal): ";
              cin >> static_cast<double>(smaller_unit);
              cin.sync();
              break;
       }
}

void convert(int bigger_unit, double smaller_unit, int& new_bigger, double& new_smaller, ConversionType conversion)
{
       const double LENGTH_CONV = .3048,  SM_L_CONV = 2.54,
                           WEIGHT_CONV = 2.2046, SM_W_CONV = 28.35;
       switch (conversion) {
       case LENGTH_TO_METRIC:
              new_bigger = bigger_unit * LENGTH_CONV;
              new_smaller = smaller_unit * SM_L_CONV;
              new_smaller += ((bigger_unit * LENGTH_CONV) - floor(bigger_unit * LENGTH_CONV)) * 100;
              while (new_smaller >= 100.0) {
                     new_bigger++;
                     new_smaller -= 100;
              }
              break;
       case LENGTH_TO_US:
              new_bigger = bigger_unit / LENGTH_CONV;
              new_smaller = smaller_unit / SM_L_CONV;
              new_smaller += ((bigger_unit / LENGTH_CONV) - floor(bigger_unit / LENGTH_CONV)) * 12;
              while (new_smaller >= 12.0) {
                     new_bigger++;
                     new_smaller -= 12.0;
              }
              break;
       case WEIGHT_TO_METRIC:
              new_bigger = bigger_unit / WEIGHT_CONV;
              new_smaller = smaller_unit * SM_W_CONV;
              new_smaller += ((bigger_unit / WEIGHT_CONV) - floor(bigger_unit / WEIGHT_CONV)) * 1000;
              if (new_smaller >= 1000.0) {
                     new_bigger++;
                     new_smaller -= 1000;
              }
              break;
       case WEIGHT_TO_US:
              new_bigger = bigger_unit * WEIGHT_CONV;
              new_smaller = smaller_unit / SM_W_CONV;
              new_smaller += ((bigger_unit * WEIGHT_CONV) - floor(bigger_unit * WEIGHT_CONV)) * 16;
              if (new_smaller >= 16.0) {
                     new_bigger++;
                     new_smaller -= 16;
              }
              break;
       }
}

void output_results(int bigger_before, double smaller_before, int bigger_after, double smaller_after, ConversionType conversion) 
{
       switch (conversion) {
       case LENGTH_TO_METRIC:
              cout << bigger_before << " feet " << smaller_before << " inches = " << bigger_after << " meters " << smaller_after << " centimeters." << endl;
              break;
       case LENGTH_TO_US:
              cout << bigger_before << " meters " << smaller_before << " centimeters = " << bigger_after << " feet " << smaller_after << " inches." << endl;
              break;
       case WEIGHT_TO_METRIC:
              cout << bigger_before << " pounds " << smaller_before << " ounces = " << bigger_after << " kilograms " << smaller_after << " grams." << endl;
              break;
       case WEIGHT_TO_US:
              cout << bigger_before << " kilograms " << smaller_before << " grams = " << bigger_after << " pounds " << smaller_after << " ounces." << endl;
              break;
       }
}