MRZ Check Digit Calculation with 7-3-1 Weights

Character values

Digits keep their value, letters A–Z map to 10–35, and the filler < counts as 0.

Extracting It Automatically

When you read this data from photos, the Passport OCR API returns it as structured JSON, and you can apply the rules on this page as validation. Try it with the free scanner.

Where This Fits

• Weighting: Multiply each character’s value by the repeating weights 7, 3, 1, add the products, and take the remainder after dividing by 10. That remainder is the check digit.
• Which fields have check digits: On a TD3 passport, the document number, date of birth, and expiry date each have a check digit, and a composite check digit covers several fields together.

Python Implementation

A minimal, dependency-free implementation you can use to validate OCR output.

Prerequisite: Python 3.9+

def mrz_check_digit(field: str) -> int:
    total = 0
    for i, char in enumerate(field):
        if char.isdigit():
            value = int(char)
        elif char.isalpha():
            value = ord(char.upper()) - 55
        else:
            value = 0  # "<" filler
        total += value * (7, 3, 1)[i % 3]
    return total % 10

print(mrz_check_digit("L898902C3"))  # ICAO sample document number -> 6

Reference Summary

Key facts from this page in structured form, for quick lookup.

{
  "Character values": "Digits keep their value, letters A–Z map to 10–35, and the filler < counts as 0..",
  "Weighting": "Multiply each character’s value by the repeating weights 7, 3, 1, add the products, and take the remainder after dividing by 10.",
  "Which fields have check digits": "On a TD3 passport, the document number, date of birth, and expiry date each have a check digit, and a composite check digit covers several fields together.."
}