Barcode won't scan? Causes and fixes
A barcode that looks fine but won't scan almost always has one of a few causes. Work through this list in order: the first ones are the most common.
1. The page was scaled when printing
"Fit to page" or "Shrink oversized pages" in the print dialog resizes the whole sheet, which makes bars thinner and moves labels off position. Print at 100% / Actual size, and check the printer's own driver settings as well as the PDF viewer's.
2. No quiet zone
Linear barcodes need clear space on both sides, called the quiet zone. Without it the scanner can't find where the barcode starts. Code 128 needs about 10 times the narrowest bar's width on each side; EAN-13 and UPC-A need similar margins. Text, label edges, borders or another barcode inside that space cause failures. Keep codes short enough that the barcode fits the label with room to spare.
3. The bars are too thin
Long codes on small labels squeeze each bar until the printer can't draw it cleanly. As a rough guide, keep the narrowest bar above 0.25 mm (10 mil) for office laser and thermal printers. If the barcode is too dense:
- shorten the code, for example drop a prefix that every item shares;
- use Code 128 instead of Code 39, which is shorter for the same text;
- pick a bigger label, such as Avery 5160 instead of 5167;
- or switch to a Data Matrix or QR code if your scanners read 2D codes.
4. Low contrast
Scanners need dark bars on a light background. Black on white works best. Red bars, light grey, faded toner, or printing on kraft, clear or coloured labels can all fail, because many scanners use red light and see red as white. Inverted barcodes (light bars on dark) are not read by many scanners.
5. Thermal printer settings
On direct-thermal and thermal-transfer printers, too little heat makes broken bars and too much makes them bleed together. Adjust the darkness (heat) and lower the print speed. Also make sure the label size in the driver matches the roll, for example 2×1 in or 50×25 mm, and that the PDF page is the label size, so the driver doesn't scale it. The thermal label generator makes one PDF page per label for this reason.
6. The barcode is printed across a label edge
If the labels on a sheet are slightly offset, a barcode can be cut by the label edge or lose its quiet zone. Print one page on plain paper, hold it against a label sheet, and use the printer offset setting to move everything into place.
7. Wrong check digit or wrong type
EAN-13, UPC-A and ITF-14 end in a check digit. A number with a wrong check digit either won't encode or scans as a different product. Make sure the barcode type matches what the scanning system expects: a till that looks up EAN-13 codes won't find a Code 128 SKU.
8. The scanner isn't set up for that type
Many scanners ship with some types turned off, often Code 39, ITF and Data Matrix. Laser and linear CCD scanners can't read QR codes or Data Matrix at all. Check the scanner's manual for its configuration barcodes, and enable the type you use. If the scanner reads the code but adds or drops characters, check its prefix and suffix settings and the keyboard layout.
9. Scanning conditions
Glossy laminate under bright light, curved surfaces such as bottles, and holding the scanner at the wrong distance all cause trouble. Tilt the scanner slightly so it isn't straight on to a glossy label, and on curved items orient the bars so they run around the curve rather than along it.
Test before printing the whole batch
Print a single page, scan several labels with the scanner or phone you'll really use, and only then print the rest. It takes two minutes and saves a pack of label sheets.