Check Digit
A check digit is a character computed from the other data characters and appended to the barcode so a scanner can detect read errors instead of silently accepting corrupted data.
What You Need to Know
Different symbologies and numbering systems use different algorithms. EAN/UPC uses modulo 10 with alternating weights of 3 and 1; Code 128 uses modulo 103 over character values; Code 39 can carry an optional modulo 43 check character; MSI Plessey offers several recursive Mod 10/11 schemes. In every case the scanner recomputes the sum after decoding and rejects the read when the result does not match the transmitted check digit.
A check digit is your last line of defense against substitution errors — a bar misread as a neighboring value, a smudged digit, or a mistyped number. It cannot correct the error, only detect most of it. Numbering authorities such as GS1 build the check digit into the identifier itself (the 13th digit of an EAN-13), so encoders calculate it automatically and you should never type it as part of your data.
Where Check Digit Applies
The 13-digit retail point-of-sale barcode used on consumer products worldwide outside North America.
Generate EAN-13 →The original 12-digit retail barcode of the United States and Canada, and direct ancestor of EAN-13.
Generate UPC-A →A high-density, variable-length linear barcode that encodes all 128 ASCII characters, and the backbone symbology of modern logistics.
Generate Code 128 →The book trade's EAN-13 — an ISBN-13 product number, usually with a 5-digit price add-on.
Generate ISBN →A digits-only inventory barcode with a single Mod 10 check digit, descended from the Plessey code.
Generate MSI Plessey (Mod 10) →