convert unittype revit api
public static double ConvertFromInternalUnits(double value, string forgeUnit)
{
if (string.IsNullOrWhiteSpace(forgeUnit))
throw new ArgumentNullException(nameof(forgeUnit));
var dut = new Autodesk.Revit.DB.ForgeTypeId(forgeUnit);
var result = Autodesk.Revit.DB.UnitUtils.ConvertFromInternalUnits(value, dut);
return result;
}