QR code size for a poster: how big does it need to be?

Posters are the one case where distance binds, not data

Everywhere else, the question is how small a code can go before the printing press and the phone camera stop resolving its individual squares. On a poster that floor is irrelevant. A 70 mm code has modules nearly two millimetres across, four times the size anything needs, so it will never be too fine to print.

What decides a poster code is how far away the reader is standing. Nobody kneels at a conference. They walk up until the poster is comfortable to read, lift a phone, and if nothing happens in two seconds they walk on.

So size for the distance. A code reads from about ten times its own width, which makes the arithmetic easy: a 50 mm code works at half a metre, an 80 mm code at eight tenths of a metre, a 200 mm code across a room.

The ten to one rule, and when it is wrong

Ten to one assumes a code of about 29 modules, which is what a normal link produces. The rule is really about the module, not the code, and a denser code has smaller modules at the same printed width.

Double the module count and you halve the distance. A 70 mm code carrying a short link reads from about 700 mm. Put a long cloud sharing URL in the same 70 mm square, with its parameters and its forty character file id, and the code goes from 29 modules to 45. Its squares are a third smaller, and it now wants the reader at 500 mm, which at a poster board means leaning in.

This is why a poster code that failed at the conference often scans perfectly on the desk afterwards. Nothing was wrong with the print. The reader was simply never that close.

Sizes for the standard poster formats

These assume a code carrying a short link and a reader who walks up to the board. If your link is long, either shorten it or add a third to the width.

• A0, 841 by 1189 mm: 70 mm minimum, 90 mm comfortable
• A1, 594 by 841 mm: 60 mm minimum, 80 mm comfortable
• A2, 420 by 594 mm: 50 mm minimum, 65 mm comfortable
• 36 by 48 inch US research poster: 70 to 90 mm, the same as A0
• Roll up banner people will not approach: 200 mm, because at two metres nothing smaller works

The mistake that ruins most poster codes

Almost every failed poster code fails for the same reason, and it is not the code. It is the scale it was designed at.

Research posters are usually built in presentation software at whatever page size was to hand, then sent to a large format printer to be fitted to the real sheet. If you designed at A1 and it prints at A0 everything doubles and the code is fine. If you designed at A0 and the print shop fits it to A1 to save money, your careful 70 mm code arrives at 49 mm, and the one that was already marginal is now unreadable.

Two habits fix it. Design at the size it will print, or at an exact half or quarter of it so the scaling is obvious. And place the code as vector artwork, an SVG, EPS or PDF, rather than a screen resolution PNG. A vector code stays crisp at any size; a 72 dpi image blown up to A0 turns every module edge to mush, and mush is what a camera cannot read.

Fabric and roll up posters

Fabric posters travel folded in a suitcase, which is why they have taken over from tubes, and folding is exactly what a QR code cannot survive. A crease through the pattern breaks the timing line, and a crease through a corner square removes one of the three markers a scanner uses to find the code at all.

If the poster is fabric, put the code where the folds will not cross it, which in practice means away from the centre lines and away from the quarter lines. Make it larger than you would on paper, because the weave softens every edge. And hang it early so the wrinkles drop out before anyone tries to scan it.

Dye sublimation on fabric also spreads ink further than paper does. Treat it like uncoated stock: give the code more room than the arithmetic says it needs.

Where to put it on the board

A poster board usually puts the top of an A0 sheet somewhere above two metres, which means the upper third of your poster is above the eye line of most of the people reading it, and well above comfortable phone height.

Keep the code in the lower two thirds. Bottom right, next to the conclusions or the contact details, is where readers are already looking when they decide they want to keep something.

Leave the blank margin around it. Four modules of clear space on every side is part of the code, not padding, and a caption or a border crowding the edge is a common reason a code that is the right size still fails.

What the code should carry

Shorter is better, because every character costs modules and modules cost scanning distance. A DOI link is short and permanent and makes an excellent poster code. A cloud sharing URL with a long file id is the worst case, and it is also the one most likely to stop working the week after the conference.

If you do not yet know where the code should point, or you want to swap the preprint for the published paper later, use a tracked code. The printed pattern stays fixed and short while the destination changes behind it, which is the only way to fix a poster after it has been printed.

Whatever you choose, check the finished file rather than the code on its own. Most poster codes are broken by the poster, not by the generator: a scale change, a low resolution placement, a background colour behind the pattern, a fold. Send the final PDF through the checker and it will tell you what the phone will see.