A QR tag for the collar: what to put in it and what size

By Tiehl Roberto Matías · 6 min read · Updated 27 October 2026

Leer en español

A tag with a code on the collar is one of the most useful things you can print: if the dog gets lost, whoever finds it calls with one gesture. But a tag is small, and what people want to put inside does not always fit. Here is the measurement of what fits, what does not, and how to make it last hanging off a dog.

What to put in the code

Whoever finds a lost dog wants to do one thing: tell the owner. The code has to get them there in as few steps as possible, and there are three ways of doing it.

The phone number, with tel:. You type it into the generator's text field like this: tel:+5493735401234, with the country code in front. Scanned, the camera offers to call that number. It needs no internet and no app, and it is the shortest of the three, so it is the one that leaves the modules biggest.

WhatsApp, with wa.me. It opens a chat with your number. It has one real advantage: whoever finds the dog can send you a photo and a location. But it asks for data and for having WhatsApp, and it takes up a little more room. The exact format of the link is in the WhatsApp guide.

A page of your own. A short link to a page with a photo of the dog, your phone number and whatever needs knowing: whether it is on medication, whether it is nervous. It is the only one of the three you can change without reprinting the tag: if your number changes, you change the page. In exchange, if the page stops existing, the tag stops working. If you pick this one, make it a page you control and will keep up, not a free link shortener.

What fits at each size

The piece for this in the generator is the keychain, and its minimum size is 35 mm wide, 43 tall counting the tab with the hole. On that piece the code takes a 29 mm square. This is what each content gives, asking for correction H and with the 0.8 mm per module minimum a 0.4 nozzle needs:

Content	Characters	Modules	At 35 mm	At 40 mm	Smallest tag
Phone number with tel:	18	29	0.88 mm	1.03 mm	35 mm
Short page of your own	23	29	0.88 mm	1.03 mm	35 mm
WhatsApp	27	33	0.78 mm	0.92 mm	36 mm
Plain text with the number	43	37	0.71 mm	0.83 mm	39 mm
WhatsApp with a message	57	41	0.64 mm	0.76 mm	43 mm
Minimal vCard	75	49	0.55 mm	0.64 mm	50 mm

Three things come out of this table. The phone number and a page of about twenty characters give exactly the same result, because they land on the same version of the code: you can choose between them for what they do, not for the size. WhatsApp misses 35 mm by two hundredths, so if you want it, take the tag up to 36 or 40. And the pre-written message, that «Found Milo» that reads so well, costs eight millimetres of tag: on a big dog that is neither here nor there, on a medium one it is already in the way.

The vCard, which is what many people try first, makes no sense here: it asks for a 50 × 58 mm tag, which is more a plate than a tag, and besides, the camera opens it as a contact to save, which is not what somebody who has just found a dog wants to do.

The correction level on a piece that gets scratched

Dropping the correction from H to M makes the modules grow: WhatsApp would fit in 35 mm with 0.88 mm modules. It is tempting, but on a tag it is not worth it.

A tag gets dragged along the ground, bangs into the food bowl and rubs against the buckle every day. Level H is the one that lets it read with close to 30 % damage, and on this piece that margin is not theoretical: it is being used up. If you want WhatsApp on a small tag, better to win the millimetre by making the piece bigger than by taking away margin. The guide to the levels explains what each one recovers.

The number written out as well

The generator lets you add a line of text under the code, up to 26 characters. On a tag, use it for the phone number written out: 3735 40-1234. It helps whoever cannot or does not know how to scan, whoever has a flat battery, and whoever simply prefers to dial.

It costs 8 mm of height: the 35 mm tag goes from 43 to 51 mm. And you need to know how it comes out: at that size the digits are 3 mm tall with a 0.6 mm stroke, less than the two nozzle passes it would take to come out crisp. It prints as a single pass and reads up close, but with soft edges. For a clean number the tag would have to be 50 mm, which is a lot of tag. The guide to text on the piece has the sum for each size.

If the extra height is a nuisance, the number can be written on the other face with a permanent marker, though it rubs off over the months.

Making it survive hanging off a dog

The material. PETG rather than PLA. It is going to get wet, sit in the sun in the car and take knocks, and PETG stands up to all three without turning brittle. It loses some detail, which on a tag with modules of nearly a millimetre does not show.

The thickness. The 3 mm base the generator uses by default is a lot for a collar: the tag is heavy and it rattles. At 2 mm it holds up with room to spare. And the relief at 0.6 mm, which is less than the generator gives you, and for a reason.

Wear works in your favour. On a two-colour piece made with this generator the code is the second colour through its whole height, not just at the surface. When rubbing files the relief down, the code loses height but not colour, so it keeps reading until the relief disappears altogether. That tempts you to raise it to 1 mm so it lasts longer, but with 0.88 mm modules a 1 mm relief only reads up to about 24° of tilt: its walls hide the light modules as soon as the phone is not head on, and somebody crouching to look at a dog's collar almost never is. At 0.6 mm it holds up to about 36°. The sum is in the relief guide. An engraved and painted tag, on the other hand, loses the paint at the edges and the contrast goes sooner. The two techniques are compared in the engraving guide.

The ring. Hang it with a closed keyring ring, not an S-hook: the hook opens with a tug and the tag gets lost on exactly the day the dog runs off. The tab, printed flat, takes the tug well, because the layers lie along the pull and there is no layer bond to come apart.

Small dogs and cats

For a cat or a two-kilo dog, a 35 × 43 mm tag is too much. Commercial tags for them are around 25 mm, and there the problem is not the design but the nozzle.

The generator does not go below 35 mm, but the slicer can scale the piece. If you scale it in width and length only, leaving the height alone so the relief stays a millimetre, this is what you are left with, with the phone number inside:

Tag	Module	0.4 nozzle	0.2 nozzle
30 mm	0.75 mm	No	Yes
25 mm	0.63 mm	No	Yes

With a 0.2 mm nozzle, which asks for a module of at least 0.4, both work. With the 0.4, neither does, and there is no way of cheating: the modules come out rounded and joined together. If you do not have a fine nozzle, for a cat the honest option is a traditional tag with the number engraved. The minimum size per nozzle is explained in its own guide.

And before putting it on the animal, scan it. With a phone that is not yours, hanging from the collar and at the distance somebody would crouch to look at it. It is the usual last test, and on this piece it is the one that matters.