Receipts: The Thermal-Printer Constraint
Receipt QR codes — for feedback surveys, digital warranties, e-invoices, or loyalty signup — are scanned from 15 to 25 cm, usually within minutes of printing. The binding constraint is not distance but the printer: most point-of-sale printers are direct-thermal devices at 203 DPI, which renders the small, sharp squares a QR code needs only approximately. A code that looks crisp in the design file can emerge from a tired thermal head as a gray smudge.
The 10:1 Rule of Thumb and the DPI Floor
At 20 cm, the rule of thumb suggests 20 mm. But on a 203 DPI thermal printer, each dot is about 0.125 mm wide, and modules need several clean dots of width to survive the heat-and-chemistry process. As practical guidance, 25 mm is the reliable minimum on thermal receipts, and the payload must stay short — a long URL forces a dense symbol whose tiny modules the thermal head cannot resolve. Short payload, large modules, generous size: that is the entire thermal receipt strategy.
Recommended Size
Print at 25 × 25 mm (about 1 inch square). Keep the encoded URL under roughly 40 characters; use a short domain with a server-side redirect. Avoid encoding vCards, Wi-Fi credentials, or long text payloads on receipts entirely — the density will defeat the printer. If your POS software lets you set the QR module size, increase it rather than just scaling the graphic.
Substrate and Printing
Receipts print on thermal paper — usually BPA-free formulations these days — via direct thermal heads that darken the paper with heat. Thermal print fades with heat, sunlight, and time: a receipt left on a car dashboard in summer can become unreadable in days. Receipt codes therefore have a limited reliable lifespan; they are for prompt actions (rate your visit, download your e-receipt, join the loyalty program), not for links anyone needs in six months.
Placement and Practical Tips
- Keep the code clear of the tear edge and the paper's side margins — partial printing from a misaligned roll is a common and invisible failure until a customer complains.
- Test on the actual printer model with a fresh roll and a nearly-spent roll; thermal dithering degrades as heads wear.
- Print a short instruction above the code ("Scan for your e-receipt") — receipt codes are easily overlooked in a wall of transaction text.
- Position the code away from the fold if receipts get folded into bags; creases through thermal print crack the image.
- Monitor your short-URL analytics: a sudden drop in receipt scans often means a dying print head, not a marketing problem.
Common Mistakes
- Encoding the full invoice URL with session tokens and tracking parameters — it creates an unscannable dense symbol on 203 DPI hardware.
- Placing the code where the cashier's thumb, the bag staple, or the receipt spike lands.
- Assuming a code that scans fresh will scan in six months — thermal fade is real, so design receipt links for prompt action.
- Shrinking the code to save paper; the fractions of a cent saved aren't worth the failed scans.