public static int ExcelColumnNameToNumber(string columnName)
{
    if (string.IsNullOrEmpty(columnName)) throw new ArgumentNullException("columnName");

    char[] characters = columnName.ToUpperInvariant().ToCharArray();

    int sum = 0;

    for (int i = 0; i < characters.Length; i++)
    {
        sum *= 26;
        sum += (characters[i] - 'A' + 1);
    }

    return sum;
}

int result = colName.Select((c, i) =>
    ((c - 'A' + 1) * ((int)Math.Pow(26, colName.Length - i - 1)))).Sum();

int col = colName.ToCharArray().Select(c => c - 'A' + 1).
          Reverse().Select((v, i) => v * (int)Math.Pow(26, i)).Sum();

sum=0;
len=length(letters);
for(i=0;i<len;i++)
  sum += ((letters[len-i-1])-'A'+1) * pow(26,i);

sum=0;
len=length(letters);
for(i=0;i<len;i++)
  sum += ((letters[len-i-1])-'A'+1) * pow(26,i);