ToEngineeringFormat.cs

public static class ToEngineeringFormat
{
    // Used in adding the units to the return string
    private static string[] prefix_const = { "y", "z", "a", "f", "p", "n", "µ", "m", "", "K", "M", "G", "T" };

    // number: The number to convert.
    // 
    // significant_digits: The number of significant digits to return. digits should be a minimum of 3 for 
    // Engineering notation this routine will work with any number from 1 to 15
    // But when significant_digits is less than 3 the output may be in scientific notation
    //
    // units: what the number is a measure of, like "Hz", "Farads", "Tesla", etc.
    public static string Convert(double number, Int16 significant_digits = 3, string units = "")
    {
        double scale = Math.Log10(Math.Abs(number));
        if (scale < 0.0)
            scale += -3.0;

        // The + 0.001 here makes sure that we use the proper scale range by pushing the calculated range just a bit.
        Int16 power = (Int16)((scale / 3) + 0.001);
        string prefix_str = prefix_const[power + 8];
        double scale_factor = Math.Pow(10.0, (double)power * 3.0);
        double base_num = number / scale_factor;

        // Make the format specifier string - bound limit the digits first
        if (significant_digits < 1)
            significant_digits = 1;
        if (significant_digits > 15)
            significant_digits = 15;
        string format_str = "G" + significant_digits.ToString();

        string converted_str = base_num.ToString(format_str) + prefix_str + units;

        return (converted_str);
    }
    //
    public static string ConvertDecimal(decimal number, Int16 significant_digits = 3, string units = "")
    {
        return Convert((double)number, significant_digits, units);
    }
}