A QR code that scans perfectly from your monitor can fail the moment it is printed on a sign, a sticker or a box, and the whole batch goes back to the printer.

Here are the usual causes, the size rule of thumb QRnaka gives, the checks built into the generator, and a short test routine that saves reprints.

How big should you print it?

The home page of QRnaka gives two rough rules:

  • At least 2 cm a side for close-up scanning, such as business cards, labels or a menu in someone's hand.
  • About a tenth of the scanning distance: if people scan from 1 m away, make the code about 10 cm wide.

For a print shop, send SVG or PDF.

This is a starting point, not a guarantee. A code holding more data has a denser grid, so at the same printed size each module is smaller and phones must come closer. The "Size" line under the preview shows the modules per side: the bigger that number, the more a larger print or less data will help.

Why printed codes fail

  • Too small for the distance. A code that looks fine on your desk can be too small on a wall.
  • Weak contrast. Pastel dots on a pale background are hard for cameras. The generator warns below 3:1.
  • Light dots on a dark background. The site warns that many phones cannot read inverted codes. Dark on light is the safe choice.
  • No quiet zone. The QR standard (ISO/IEC 18004) asks for 4 modules of clear space; the generator starts at 4 and warns that less may fail on a busy background. Trouble starts in layout, when the border is trimmed or text crowds the code.
  • Too much data. Long links and packed contact cards make dense codes, so trim what you can. For a long link, a tracked QR code uses a short link instead; tracked codes are a member feature with a 30-day free trial, then 49 baht a month or 365 baht a year. See dynamic QR codes explained.
  • Glare. Glossy lamination, acrylic and shop windows reflect lights into the camera. Matte helps.
  • Damage. Error correction survives some stains and tears, but the three large corner squares must stay clear. See QR error correction levels.
  • Wrong scale or blur. "Fit to page" changes the size, and a small PNG blown up gets fuzzy. Use SVG, PDF or a high-resolution PNG.
  • Curves and creases. On cups, bottles or crumpled bags the code bends. Test on the real object.

What the generator checks for you

  • A "Scans" badge after every change: the code is drawn and read back on your device. Otherwise you see "May not scan"; the page's advice is more contrast, a smaller logo or simpler shapes.
  • Warnings for weak contrast, light-on-dark colours and an oversized logo.
  • A "Try damaging it" slider that adds simulated stains and tears.
  • Error correction levels L, M, Q and H, raised automatically when the size stays the same.
  • A quiet zone slider and print files: PNG up to 4096 px, SVG and PDF.

The printable card on the PromptPay QR page is checked as a whole too. Still, all of this reads the image on screen. The site's terms say printing, size, colour, material and phone cameras all vary, so test with real phones before printing many.

A test routine before the print run

  1. Print one at actual size (100%, not "fit to page"), on the same material and finish if you can.
  2. Put it where it will live, at the same height, angle and light. If you open at night, test at night.
  3. Scan with several phones, iPhone and Android, new and old, with the camera app and the apps people will use, such as a banking app for payment codes or LINE for a LINE code.
  4. Stand where people will stand.
  5. Check the result, not just the scan. The link must open the right page; a payment code must show the right payee and amount before you cancel.
  6. To see what a printed code holds, open a photo of it on the Read QR page, which reads on your device and uploads nothing.

Then order the full run, and keep the SVG or PDF for reprints.

FAQ

The page says "Scans". Do I still need to test?

Yes. The badge means the on-device reader can read the image on screen. The site itself recommends testing with a real phone before printing many.

Which file should I send to a print shop?

SVG or PDF. If they want an image, download a 2048 or 4096 px PNG.

Can I print on a coloured background?

Yes, with dark dots on a light colour, clear contrast and a white border. The generator warns you when colours are risky.

My printed code scans badly. Can I fix it?

Not on paper. Reprint it larger or with stronger contrast, and if the code is dense, cut the data first.

Next time, open the QR generator, wait for the "Scans" badge, try the damage slider, and print one test copy at actual size before the rest.