| Frequently Asked Questions : Bar Code Technology FAQs : Mechanics
What is a Check Digit?
A check digit is used to check that the data is read correctly. Different symbologies apply different formulas to the encoded numbers to yield a single digit. That digit is the check digit. That check digit is usually added to the end of the already encoded numbers.
The computer checks that the numbers were read correctly by comparing the check digit it calculates against the check digit it read.
For example: When encoding the ZIP code 311215-12352 into POSTNET, the check digit is the total of the numbers subtracted from the next higher multiple of ten.
1 + 1 + 2 + 1 + 5 + 1 + 2 + 3 + 5 = 21
30 is the next higher multiple of ten
30 - 21 = 9
the check digit = 9
In the POSTNET bar code the numbers to be encoded would be 311215123592. The bar code reader reads 311215123592. Reading a check digit of 392 and calculating a check digit of 392 from the other numbers it read, the bar code reader now is doubly certain that the numbers were correctly decoded.
Bar code technology is millions of times more accurate than typing when it comes to entering information into the computer. Check digits make the systems even more accurate. Tests have shown that operators may do 10,000,000 entries between errors when using check digits.
Updated: 05/12/2003
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