The method
Digits use their numeric values, letters A–Z use values 10–35 and < uses zero. Apply repeating weights 7, 3 and 1, sum the products, then take the remainder after division by 10. The ICAO specification defines this calculation. ICAO Doc 9303, Part 3: check-digit method
A match is not an authenticity result
Different inputs can return the same digit. A match establishes only that the compared digit agrees with this arithmetic on the string supplied. The tool does not check the correct field placement, issuer, document edition, calendar rules, chip data or holder identity.