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What a check digit is
Many everyday numbers carry a built-in check digit: one digit calculated from all the others, so a typing slip usually breaks the sum. Card numbers and IMEIs use the Luhn algorithm; books and retail barcodes use the ISBN/EAN/UPC weighted sums. These algorithms are public — the same maths runs in every till, library system, and spreadsheet.
Try it
79927398713 is the textbook Luhn example and validates. Change its last digit and it fails. 9780306406157 is a valid ISBN-13. Deliberately mistyping one digit of any of them shows how the check catches ordinary transcription errors — transposed or mistyped digits are exactly what these sums were designed to catch.
What this cannot do
A number can pass every check here and still be invented, expired, cancelled, or someone else's. This tool can never confirm a card is live, a book is registered, or a product exists. Only the issuer can do that.
Common questions
A number passed — does that mean it's genuine?
No. It means the digits are internally consistent. Scam and test numbers are often built to pass check digits. Genuine confirmation only ever comes from the issuing organisation.
Why does an IMEI / card number with spaces still work?
Spaces and hyphens are stripped before the maths — they are formatting, not part of the number.