How the barcode works
A UPC or EAN barcode stores only a number. The product name, price and stock live in the shop's system and are looked up by that number at the register. The last digit is a check digit: a scanner recalculates it and rejects the read when it does not match, which catches most misreads and typing errors.
Calculating the check digit
- Take the digits without the check digit.
- Starting from the rightmost digit, multiply digits by 3, 1, 3, 1 and so on.
- Add the results.
- The check digit is the amount needed to reach the next multiple of 10 (0 when the sum already ends in 0).
For 03600029145: 0×3 + 3×1 + 6×3 + 0×1 + 0×3 + 0×1 + 2×3 + 9×1 + 1×3 + 4×1 + 5×3 = 58, so the check digit is 2 and the full UPC-A is 036000291452. The generator shows this calculation for every number.
UPC-A, EAN-13 and EAN-8
| Format | Digits | Used in | Size at 100% |
|---|---|---|---|
| UPC-A | 12 | United States and Canada retail | 37.29 × 25.93 mm |
| EAN-13 | 13 | Rest of the world, books (ISBN), most marketplaces | 37.29 × 25.93 mm |
| EAN-8 | 8 | Small packs where an EAN-13 does not fit | 26.73 × 21.31 mm |
A UPC-A is an EAN-13 with a leading zero: 036000291452 and 0036000291452 are the same product, and scanners outside North America read UPC-A without changes.
Where barcode numbers come from
The generator draws a correct barcode for any number, but it does not issue numbers. For products sold through retailers or marketplaces, the number has to start with a company prefix licensed from GS1, the organization that runs the UPC and EAN system. Retail chains and Amazon check the prefix against the GS1 database.
Print size and quality
The nominal bar width, called X, is 0.33 mm. Retail scanners read sizes from 80% to 200% of nominal, and the slider above keeps to that range. The SVG export carries the size in millimetres, so it prints at the chosen size without scaling.
| Size | X | UPC-A / EAN-13 width | EAN-8 width |
|---|---|---|---|
| 80% | 0.264 mm | 29.83 mm | 21.38 mm |
| 100% | 0.330 mm | 37.29 mm | 26.73 mm |
| 150% | 0.495 mm | 55.94 mm | 40.10 mm |
| 200% | 0.660 mm | 74.58 mm | 53.46 mm |
- Keep the quiet zone. The white space left and right of the bars is part of the symbol. Text or a box edge inside it makes the barcode unreadable.
- Black on white. Dark bars on a light background. Red bars do not scan, because most scanners use red light.
- Do not cut the height. Shorter bars make the barcode harder to read at an angle.
- Thermal label printers. Use the SVG, or the PNG at 300 dpi, and print at 100% scale with no "fit to page".
Label sheets from a list
The Batch tab takes a list of numbers, one per line, with an optional label text after a comma. A product export from Square, Zettle, Shopify or WooCommerce can be pasted as it is once the barcode and SKU columns are next to each other. Each line gets its own barcode; missing check digits are added and wrong ones are flagged with the line number.
The sheet prints on A4 with 21 labels of 63.5 × 38.1 mm (3 columns, 7 rows), the most common office label layout. The ZIP export contains one SVG per barcode, and the CSV lists every full number with its check digit for importing back into the shop system.
Questions
Is the generator free?
Yes. No account, no watermark and no limit. Everything runs in the browser and no number is sent anywhere.
Can these barcodes be used on Amazon?
The barcode itself is standard. Amazon accepts it only when the number comes from a GS1 company prefix registered to the brand.
12 digits: UPC-A or EAN-13?
A 12-digit number with a valid check digit is a full UPC-A. In Batch mode with format set to Auto, a 12-digit number whose last digit does not fit as a UPC check digit is treated as an EAN-13 without its check digit.
Do barcode numbers need to match the SKU?
No. The SKU is the shop's own code and the barcode number is the product's global ID. Inventory sync tools such as StockEven match products by SKU, so the two can differ.