Work through the check

Four things a result can tell you

1. A matching final digit

Enter 978-0-306-40615-7. Removing the plain hyphens gives thirteen digits. The first twelve weighted products total 93. Since 93 leaves remainder 3 on division by 10, the required final digit is 7. The entered 7 matches. The result says nothing about a title or edition because no metadata lookup occurs.

2. A mismatch worth checking at the source

Enter 9780306406158. The same first twelve digits still require 7, but the entry ends in 8. Bookdigit reports a mismatch and keeps the original entry visible. It does not assume the final digit is the only typo. A wrong earlier digit could instead be responsible.

3. A 979 mathematical example

Enter 9791234567896. The first twelve weighted products total 134, so remainder 4 requires final digit 6. The checksum matches. This is an arithmetic illustration only: assignment, publisher and a corresponding book are not established. No ISBN-10 conversion is attempted.

4. A number outside this checker's prefix scope

An identifier beginning 9790 receives an input explanation rather than an ISBN result. The 979-0 prefix is used for ISMN printed-music identifiers. A checksum match alone would not make it an ISBN, so the prefix rule is applied first.

After editing

Calculate an example, then change any digit or separator. The previous diagnostic disappears and export buttons become unavailable. Choose Check checksum to create a fresh diagnostic for the current entry. Restore example replaces the input with the starting example and also clears the result.

Try the local checker. Text and JSON exports contain the entered identifier and the diagnostic; share them only if you want to share that information.