Your passport's machine-readable lines check their own arithmetic

A digit that tests the characters before it

In the machine-readable zone—the two lines of letters, numbers, and angle brackets at the bottom of a passport’s identity page—the passport number is followed by a check digit. That last digit is calculated from the characters before it, giving border-reading software a quick way to test whether the number was captured consistently.

The calculation turns each character into a value: numerals keep their value, while letters are assigned values from 10 for A through 35 for Z. The system multiplies those values by a repeating sequence of weights—7, 3, 1—adds the results, then takes the last digit of the sum. That remainder is the check digit.

How a typo changes the result

Suppose a scanner or a person mistakes one character for another. The altered character changes the weighted sum, so the calculated check digit will usually differ from the digit printed in the passport. The software can flag the mismatch immediately, without first asking a database whether that passport number exists.

This is a consistency check, not an identity check: a matching digit shows that the characters fit the prescribed arithmetic, but it does not by itself prove that a passport is genuine or belongs to its holder. Its practical value is speed. A tiny calculation can catch many transcription errors before they travel further into a border system.