switch only uses builtin types.
Code:
#include <iostream>
int main()
{
using namespace std;

	double num1, meters, feet, kilos, pounds, liters, gallons, kms, miles;
	string option;

	cout << "English/Metric Conversion Calculator v1.0" << endl;
	cout << "-----------------------------------------" << endl;
	cout << "1. Convert feet to meters" << endl;
	cout << "2. Convert meters to feet" << endl;
	cout << "3. Convert lbs. to kilograms" << endl;
	cout << "4. Convert kilograms to lbs." << endl;
	cout << "5. Convert gallons to liters" << endl;
	cout << "6. Convert liters to gallons" << endl;
	cout << "7. Convert square miles to square kilometers" << endl;
	cout << "8. Convert square kilometers to square miles" << endl;
	cout << "9. Quit" << endl << endl;
	
	do							// makes sure the loop is ran at least once
	{	
	cout << endl << "Please select the conversion type you want to perform:" << endl;

	getline(cin, option);


	if(option == "1")
	{
		cout << endl << "Enter the number of feet to convert to meters: ";
		cin >> num1;
		meters = num1 * 0.3048;
		cout << endl << num1 << " feet is equal to " << meters << " meters." << endl;
	}
	else if(option == "2")
	{
		cout << endl << "Enter the number of meters to convert to feet: ";
		cin >> num1;
		feet = num1 * 3.2808399;
		cout << endl << num1 << " meters is equal to " << feet << " feet." << endl;
	}
	else if(option == "3")
	{
		cout << endl << "Enter the number of pounds to convert to kilograms: ";
		cin >> num1;
		kilos = num1 * 0.45359237;
		cout << endl << num1 << " lbs. is equal to " << kilos << " kilograms." << endl;
	}
	else if(option == "4")
	{

		cout << endl << "Enter the number of kilograms to convert to pounds: ";
		cin >> num1;
		pounds = num1 * 2.20462262;
		cout << endl << num1 << " kilograms is equal to " << pounds << " lbs." << endl;
	}
	else if(option == "5")
	{
		cout << endl << "Enter the number of gallons to convert to liters: ";
		cin >> num1;
		liters = num1 * 3.7854118;
		cout << endl << num1 << " gallons is equal to " << liters << " liters." << endl;
	}
	else if(option == "6")
	{
		cout << endl << "Enter the number of liters to convert to gallons: ";
		cin >> num1;
		gallons = num1 * 0.264172051;
		cout << endl << num1 << " liters is equal to " << gallons << " gallons." << endl;
	}
	else if(option == "7")
	{
		cout << endl << "Enter the number of square miles to convert into square kilometers: ";
		cin >> num1;
		kms = num1 * 2.58998811;
		cout << endl << num1 << " square miles is equal to " << kms << " square kilometers." << endl;
	}
	else if(option == "8")
	{
		cout << endl << "Enter the number of square kilometers to convert into square miles: ";
		cin >> num1;
		miles = num1 * 0.386102159;
		cout << endl << num1 << " square kilometers is equal to " << miles << " square miles." << endl;
	}
	else
	{
		cout << "That is an invalid option.  Please choose again." << endl;
	}
	}
	while(option != "9");			// keep looping to ask user as long as '9' isn't chosen
	cout << "Thank you for using Conversion Calculator v1.0!" << endl;
return 0;
}
Looks like Yarin gets the last laugh.