Fabrice's weblog

If for some reason you need to convert an ISBN-13 to ISBN-10 (one being that Amazon doesn't support ISBN-13 in product affiliate links) then you need not only to remove the first three characters. You also need to recompute the checksum.

Here is a piece of code that I wrote to handle this conversion:

public static String Isbn13to10(String isbn13)
{
  if (String.IsNullOrEmpty(isbn13))
    throw new ArgumentNullException("isbn13");
  isbn13 = isbn13.Replace("-", "").Replace(" ", "");
  if (isbn13.Length != 13)
    throw new ArgumentException("The ISBN doesn't contain 13 characters.", "isbn13");

  String isbn10 = isbn13.Substring(3, 9);
  int checksum = 0;
  int weight = 10;

  foreach (Char c in isbn10)
  {
    checksum += (int)Char.GetNumericValue(c) * weight;
    weight--;
  }

  checksum = 11-(checksum % 11);
  if (checksum == 10)
    isbn10 += "X";
  else if (checksum == 11)
    isbn10 += "0";
  else
    isbn10 += checksum;

  return isbn10;
}

I use this to create the Amazon affiliate links on Clair de Bulle, the famous French comics website of my wife. Yes, it's the holidays season, so I have to work for her ;-)