QR codes on postcards: size, placement and what a scan tells you

A QR code on a postcard works when four things are true: the link inside it is short, the code is printed dark on a light background, there is a blank margin around it, and it is large enough for a phone camera to read at arm's length. Get those right, test one printed card with two different phones, and print a phone number or web address beside the code for people who will not scan. The rest of this article explains each point and what a scan can and cannot tell you.

How big should a QR code be on a postcard?

A QR code is a grid of small squares called modules. What matters for scanning is the size of each module, not only the size of the whole code. DENSO WAVE, the company that invented the QR code, explains that the printed size is the number of modules multiplied by the module size, plus the margin.

The number of modules depends on how much data you store. QR codes come in versions 1 to 40. Version 1 is 21 by 21 modules, and each version adds four modules per side, up to 177 by 177. More data means more modules, so at the same printed size each module gets smaller and harder to read.

There is no single official minimum size for a postcard. As a starting point, we suggest about 1 inch (25 mm) square including the margin, and larger if you have room. That is our rule of thumb, not a standard. Here is the arithmetic behind it: a version 3 code, the size DENSO WAVE uses in its example for 50 letters and numbers, is 29 modules wide, plus 4 modules of margin on each side, which makes 37. One inch is 25.4 mm, and 25.4 ÷ 37 is about 0.69 mm per module. DENSO WAVE says a common laser printer can print modules down to about 0.17 mm, so 0.69 mm leaves plenty of room for print quality and for an ordinary phone camera. Then test, because the proof is the only real answer.

What is the quiet zone?

The quiet zone is the blank margin around the code. Scanners use it to find where the code starts and ends. DENSO WAVE states that a QR code needs a margin four modules wide on all sides. Do not let text, photos, borders or the edge of the card enter that margin. If your design tool crops the white border off the code image, add it back.

What colors and contrast work?

The safest choice is black on white. DENSO WAVE's guidance is to print QR codes in black on a white background, because reading depends on the difference in light reflected by the dark and light areas. If you use brand colors, keep the modules dark and the background light, and test the result. The same page warns that glossy surfaces and transparent materials can make a code hard to read, so if your card has a gloss coating, check a coated proof under indoor light.

Be careful with logos and artwork inside the code. A QR code has built-in error correction, which lets it be read when part of it is dirty or damaged. DENSO WAVE lists four levels and says level M, which can restore about 15% of the code, is the one most often chosen. Putting a logo over the code uses up that safety margin, and DENSO WAVE warns that a deformed code may fail to read or read slowly. A plain code is more reliable.

Where should the code go on the card?

• Put it where a thumb will not cover it and where the card lies flat, not across a fold or right at the trimmed edge.
• Keep it away from the mailing address. USPS reserves a strip in the lower right corner of the address side, 4-3/4 inches wide and 5/8 inch high, for its own barcode (DMM 202.5.1).
• Place a short instruction next to it that says what happens: "Scan to get your quote" is better than "Scan me".
• Use one code per card. Two codes side by side confuse cameras and readers alike.

Why use a short link?

A short link stores less data, so the code needs fewer modules and each module prints larger. A long address with tracking tags can push the code up several versions and make it dense.

A short link that redirects has a second benefit. The printed code points to the short address, and the short address forwards to your real page. If the page changes after the cards are printed, you update the redirect and the printed cards still work. The risk is the other side of the same coin: if the short link service stops working, so does every card. Use a link you control or a provider you trust to keep it running.

How do I test a QR code before printing?

1. Print the card at actual size on paper close to the final stock. A code on a screen is not a fair test.
2. Scan it with at least two phones, one iPhone and one Android, using the built-in camera app.
3. Scan in ordinary indoor light, at arm's length, and at a slight angle.
4. Check that the page that opens is the right one, loads quickly on mobile data, and is easy to use on a small screen.
5. If the printer sends a physical proof, repeat the test on it before you approve the run.

What can a scan tell you, and what can it not?

When someone scans, their phone opens a web address. Your server or link service can record that the address was opened, when, and some general details the phone's browser sends, such as the type of device. That is all a scan is: a page request.

If every card carries the same code, a scan is anonymous. You know someone scanned, not who. If each card has its own code, a scan can be tied to the card it was printed on, and so to the address that card was mailed to. Even then, you know which card was scanned, not which person in the household scanned it, and the same person may scan twice. A scan becomes a lead only when the person fills in a form, calls or books.

What about people who cannot scan?

Not everyone can or wants to use a QR code. Some people have low vision, some have older phones, and some simply prefer to call. Print a phone number and a short, easy-to-type web address near the code, in text large enough to read. This also protects you if the code is damaged in the mail.

On PostalNest, each card gets its own QR code automatically, so a scan is tied to one recipient. You choose where the code leads: a lead form, your website, a phone call or a booking page.