How Barcodes Read A Product

A barcode is just a font for machines. The width of each bar and gap encodes digits; the scanner reads the bars and turns them back into a number.

The pattern

A standard UPC-A barcode has 59 bars of varying widths — black bars and white gaps. The widths aren't random: each digit from 0–9 is encoded as a unique pattern of two bars and two gaps, each 1, 2, 3, or 4 units wide. The pattern is the same idea as Morse code, drawn in rectangles.

The scanner's job

• A laser (or, in modern scanners, a camera) sweeps across the bars.
• Dark bars absorb light; white gaps reflect it.
• A sensor measures the timing of reflections — long flashes = wide gaps, short flashes = narrow gaps.
• The timing pattern is decoded back into digits.

The hidden structure

A UPC isn't one random number. It's:

• 6 digits identifying the manufacturer (assigned by GS1, the global standards body).
• 5 digits for the product (chosen by the manufacturer).
• 1 final check digit, computed from the previous 11, that lets the scanner detect a misread.

If the check digit doesn't match, the scanner rejects the read and asks for another pass.

Two-way reading

A UPC is symmetrical but mirror-encoded: the left half and right half of the same digit are exact mirror images. That's why the scanner can read the code in any direction and still figure out which way is "forward."

From bars to product

The scanner sends the decoded number to the store's system. The system looks up the number in a database and returns the product name and price. The barcode doesn't contain the price — it contains only the ID. The price lives in the store's database, which is why the same product can ring up different prices in different stores.

The quiet zones

The blank space on either end of the barcode. Without them, the scanner can't tell where the code starts and stops. The whole system rests on a few millimeters of white space at each end.

A barcode is, in the end, a number painted in stripes — a font designed to be read by light at arm's length, in any direction, in a fraction of a second.