Check Digit Validator

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ISBN-13 check digit validator

Weight each digit 1 or 3 alternately from the left, sum, and require a multiple of 10.

9780306406152 fails the ISBN-13 checksum, and 13 different single-character fixes would each satisfy it — checksum arithmetic alone cannot narrow this down further.

13 equally valid single-character fixes — checksum arithmetic alone cannot narrow this down to one digit:

  • 4780306406152 (substitution, change the character at position 1 from "9" to "4")
  • 9280306406152 (substitution, change the character at position 2 from "7" to "2")
  • 9730306406152 (substitution, change the character at position 3 from "8" to "3")
  • 9785306406152 (substitution, change the character at position 4 from "0" to "5")
  • 9780806406152 (substitution, change the character at position 5 from "3" to "8")
  • 9780356406152 (substitution, change the character at position 6 from "0" to "5")
  • 9780301406152 (substitution, change the character at position 7 from "6" to "1")
  • 9780306906152 (substitution, change the character at position 8 from "4" to "9")
  • 9780306456152 (substitution, change the character at position 9 from "0" to "5")
  • 9780306401152 (substitution, change the character at position 10 from "6" to "1")
  • 9780306406652 (substitution, change the character at position 11 from "1" to "6")
  • 9780306406102 (substitution, change the character at position 12 from "5" to "0")

…and 1 more.

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=978-0-306-40615-7&format=isbn13'