Unit Converter  Manual                 Creating New Primary and Secondary Conversions:     If you choose to add your own custom conversions to Unit Converter, it is very important that you understand how Unit Converter calculates conversions.  This program allows you to calculate over 100,000 different conversions among over 1,000 different measurement units, but that does not mean that there are 100,000 different conversion factors programmed in.  Rather, a small number of measurement units are designated as Primary conversion units and all other units in that measurement class are designated as Secondary conversion units.  The conversion calculation then becomes a three-step process: Your Input unit gets converted to a Primary conversion unit. This Primary conversion unit gets converted to another Primary conversion unit. This second Primary conversion unit gets converted to your Output unit.     As an example, imagine that you want to convert from Kilometers to Miles.  The Length measurement class has 77 units, but only two of these are Primary conversion units.  The two primary conversion units for Length are Feet and Meters, and all units in the Length measurement class have a conversion factor to either Feet or Meters.  Your conversion from Kilometers to Miles would go as follows: Kilometers gets converted to Meters (or multiplied by 1,000). Meters gets converted to Feet (or multiplied by 3.280839895013). Feet gets converted to Miles (or divided by 5,280).     If you wanted to convert from Centimeters to Inches, the conversion would follow the same three-step process, with both Centimeters and Inches initially being converted to Meters and Feet: Centimeters gets converted to Meters (or multiplied by .01). Meters gets converted to Feet (or multiplied by 3.280839895013). Feet gets converted to Inches (or multiplied by 12).     Conversions don’t have to convert between different primary conversion units.  If you want to convert between Miles and Inches, the same three-step process would apply but the Primary conversion factor would be 1: Miles gets converted to Feet (or multiplied by 5,280). Feet gets converted to Feet (or multiplied by 1). Feet gets converted to Inches (or multiplied by 12).     This three-step conversion process is very useful in two respects.  First, it makes the initial programming much simpler and database size much smaller, in that the number of conversion factors necessary in the database is reduced from over 100,000 to a little over 1,100.  Second, it gives the user the ability to add a measurement unit to a measurement class, enter a single conversion factor for that unit, and then be able to convert from that unit to all other units in that class.  For example, if a user wanted a unit called a “half-meter”, then they could enter in a conversion factor for Half-meter to convert it to an appropriate primary conversion unit (Meter), and then they would be able to convert Half-meters to and from all 77 other length units.     Unit Converter allows you to create new measurement classes with associated Primary Conversion units.  If you plan on creating a new measurement class with several Secondary conversion units in it, consider carefully what would be your most appropriate Primary conversion units.  If you can break down your secondary measurement units into two broad groups, then consider assigning a representative unit from each group as the two Primary conversion units.     For example, in the Length measurement class, I identified two broad groups of Length measurements that could be generally categorized as Metric and Standard.  Most Metric units had exact conversions among themselves, as did most Standard units.  Therefore I made one of the Primary Conversion units a Metric unit (Meters) and the other Primary Conversion unit a Standard unit (Feet).  The advantage of doing it this way is that any Metric-to-Metric conversions (such as Centimeters to Kilometers) are done entirely with exact conversions (Centimeters à  Meters à  Meters à  Kilometers) and any Standard-to-Standard conversions (such as Inches to Miles) are also done entirely with exact conversions (Inches à  Feet à  Feet à  Miles).     If I had made both Primary conversion units to be Metric units, then any Standard-to-Standard conversions would have suffered some from the fact that there is no exact conversion going from Standard to Metric.  For example, if my Primary conversion units were Meters and Centimeters, and I wanted to convert from Inches to Feet, the three-step conversion would probably go as follows: Inches would be converted to Centimeters (or multiplied by  2.54, which is exact). Centimeters would be converted to Meters (or multiplied by .01, which is exact). Meters would be converted to Feet (or multiplied by 3.280839895013, which is close, but not quite, exact).     This would still give precise results, and accurate to many decimal places, but it’s still not as good as it could be.  It’s a better strategy to include all measurement units with exact conversions in a single broad group, and have a single Primary Conversion unit represent all of them.     In order to simplify this process as much as possible, this software restricts you to two Primary Conversion units for each new measurement class you create.  Any additional measurement units you add to that class must be secondary units and must be converted to one of the two primary units.