ISBN Checker
Validate and convert ISBN-10 and ISBN-13 book numbers. Extract digits for puzzle solving.
ISBN Check Digit Formulas
ISBN-10: (d₁×10 + d₂×9 + d₃×8 + ... + d₉×2 + check) mod 11 = 0
ISBN-13: (d₁×1 + d₂×3 + d₃×1 + ... + d₁₂×3 + check) mod 10 = 0
Puzzle Tips
- • ISBN digits may form coordinates
- • Check digits can verify solutions
- • The X in ISBN-10 represents 10
- • Book-themed caches often use ISBNs
What is an ISBN?
ISBN (International Standard Book Number) is a unique numeric identifier for books. ISBN-10 was used until 2007, when it was replaced by ISBN-13. The last digit is a check digit that validates the number.
ISBN Formats
ISBN-10
10 digits where the last digit is a check digit (0-9 or X, where X=10). Example: 0-306-40615-2
ISBN-13
13 digits starting with 978 or 979. The check digit is always 0-9. Example: 978-3-16-148410-0
ISBN in Geocaching
ISBNs appear in geocaching puzzles because:
- Book themes: Library or literature-related caches
- Digit extraction: ISBN digits can form coordinates
- Validation puzzles: Find the correct ISBN to solve
- Check digits: Mathematical verification element
Check Digit Calculation
ISBN-10
Multiply each of the first 9 digits by 10 down to 2, sum them, then the check digit makes the total divisible by 11.
ISBN-13
Multiply digits alternately by 1 and 3, sum them, and the check digit makes the total divisible by 10.
Puzzle Applications
- Find the book: ISBN leads to a specific book with clues
- Extract numbers: Use ISBN digits for coordinates
- Validate answers: Check digit confirms correct ISBN
- Convert formats: ISBN-10 to ISBN-13 reveals new digits