Barcode Check Digit Calculator
Calculate or verify the check digit on a barcode number.
What you provide
With or without the check digit. Without it, the digit is calculated; with it, the digit is verified.
How this calculation works
All the GS1 formats share one scheme. Working from the right, digits are multiplied alternately by three and one, the products are summed, and the check digit is whatever brings that total up to the next multiple of ten. Enter a number without its check digit and the digit is calculated; enter a complete number and it is verified against what it should be. The formats differ only in how many digits precede the check.
What the results mean
- Check digit
- The final digit of the barcode. Its whole job is to make a mistyped or transposed number fail rather than scan as a different product.
- Complete number
- The body plus the calculated check digit, ready to encode. This is what goes into a barcode generator.
- Verification
- Shown when you enter a complete number. A mismatch means at least one digit is wrong somewhere in the sequence.
Common problems and fixes
- A scanner refuses to read a barcode I generated
- Check the check digit first, since most generators calculate it for you and a number entered with its own check digit already attached ends up with two. Then check the symbology matches the digit count: EAN-13 needs twelve digits plus the check, UPC-A needs eleven, and feeding the wrong length to a generator produces something that looks like a barcode and scans as nothing.
- The barcode scans as a different product
- The number is allocated to somebody else. Barcode numbers are not free to invent: a real one starts with a company prefix issued by GS1, and a made-up number that passes the check digit will collide with whoever holds it. For internal use where numbers never leave the building, that is tolerable; for anything sold through a retailer, it is not.
Frequently asked questions
Why do barcodes have a check digit?
Because a scanner reading a damaged or partly obscured label needs a way to know it read wrong. The weighted scheme catches every single-digit error and the great majority of transpositions, so a misread fails outright rather than silently returning a different product.
Is an ISBN-13 the same as an EAN-13?
Structurally yes. An ISBN-13 is an EAN-13 with a 978 or 979 prefix, and it uses the same check digit calculation. The older ten-digit ISBN used a different scheme with a modulus of eleven, which is why it can end in an X.
Can I make up a barcode number?
Only for internal use. Retail barcodes come from a company prefix that GS1 allocates, and using an unallocated number means colliding with whoever holds it or will hold it. Retailers check this, and a product with a borrowed number tends to be rejected at the point it matters most.
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