Convert and Parse Prefix Multipliers - C#

Introduction

The two functions convert/parse number strings with prefix multipliers (Milli, Kilo, Mega, Giga, etc.).

ParsePrefix

"124.67uH" >> 1.2467e-4
"124670uH" >> 1.2467e-7

"124.67mW" >> 1.2467e-1

"124.67MW" >> 1.2467e8

"100pF" >> 1e-10

AddPrefix

0.1123123 >> "112.3123m"

112312.3  >> "112.3123K"

1000 >> "1K"

1000000000000, "B" >> "1TB"

Here is the code:

/// <summary>
/// Parses a prefix number to double. Unit is ignored.
/// Prefixes greater than 1 ([K]ilo, [G]ega etc) etc are case insensitive.
/// (However, m is milli, M is Mega)
/// </summary>
/// <param name="value"></param>
/// <returns></returns>
public static double ParsePrefix(string value)
{
    string[] superSuffix = new string[] { "K", "M", "G", "T", "P", "A", };
    string[] subSuffix = new string[] { "m", "u", "n", "p", "f", "a" };
    char sufChar = '\0';
    foreach (char c in value)
    {
        foreach (string s in subSuffix)
            if (c.ToString() == s)
            {
                sufChar = c;
                string num = value.Substring(0, value.IndexOf(sufChar));
                return Convert.ToDouble(num) / Math.Pow(1000,
                       subSuffix.ToList().IndexOf(sufChar.ToString()) + 1);
            }
        foreach (string s in superSuffix)
            if (c.ToString().ToLower() == s.ToLower())
            {
                sufChar = s[0];
                string num = value.Substring(0, value.IndexOf(c));
                double mult = Math.Pow(1000, superSuffix.ToList().IndexOf
                              (sufChar.ToString()) + 1);
                return Convert.ToDouble(num) * mult;
            }
    }
    return Convert.ToDouble(value);
}

/// <summary>
/// Selects a suitable prefix for the value to keep 1-3 significant figures
/// on the left of the decimal point.
/// </summary>
/// <param name="value"></param>
/// <param name="unit">Optional unit to be put after the prefix</param>
/// <returns></returns>
public static string AddPrefix(double value, string unit = "")
{
    string[] superSuffix = new string[] { "K", "M", "G", "T", "P", "A", };
    string[] subSuffix = new string[] { "m", "u", "n", "p", "f", "a" };
    double v = value;
    int exp = 0;
    while (v - Math.Floor(v) > 0)
    {
        if (exp >= 18)
            break;
        exp += 3;
        v *= 1000;
        v = Math.Round(v, 12);
    }

    while (Math.Floor(v).ToString().Length > 3)
    {
        if (exp <= -18)
            break;
        exp -= 3;
        v /= 1000;
        v = Math.Round(v, 12);
    }
    if (exp > 0)
        return v.ToString() + subSuffix[exp / 3 - 1] + unit;
    else if (exp < 0)
        return v.ToString() + superSuffix[-exp / 3 - 1] + unit;
    return v.ToString() + unit;
}