ISBN-10 check digit validator
Multiply each of the first 9 digits by its position weight (10 down to 2), add the check digit (weight 1, and the only position allowed to be 'X' meaning 10), and require the total to be a multiple of 11.
0306406152 is a valid ISBN-10: the check digit match.
A passing checksum only proves the number is arithmetically self-consistent. It does not prove an IBAN names an open account, an ISBN is assigned to a real book, or a card is active, unstolen, or has funds. Those require asking the bank, the ISBN registry, or the card network.
Try it: a valid example · a broken example
curl 'https://check-digit.gumballtools.com/api/v1/run?input=0-306-40615-2&format=isbn10'