What your engraver needs to know

Size

The one number that decides it

The code is square, so width is the only dimension that matters. A long thin plate is no help.

17 mm, about 5/8 inchWhat we ask for.
14 mm is the absolute minimumAnd it has no margin for error.
Below 14 mm, it cannot workNot at any price, by any engraver. This is arithmetic rather than craftsmanship.

Those figures come from the smallest square a phone camera can resolve in bad light. The symbol is 25 modules across plus a clear margin of 4 on each side — 33 in total — and each of those needs to be at least 0.4 mm, with 0.5 mm the target. 33 × 0.5 mm is 17 mm.

Bigger is better and there is no upper limit. If the piece has room, use it.

Once you have an account, your own code comes with a specification sheet carrying these same requirements alongside the actual artwork, sized for your record. This page is the public version of it.

Material

The metal

Ordered by how well it survives being worn. If your engraver offers a choice and you have no reason to prefer otherwise, take the first one.

316L stainless steel — the defaultWhat the medical-ID and body-jewelry trades use. Its molybdenum is there specifically to resist chlorides, which is what sweat is, and it takes a dark, clean mark.
Titanium — excellent, and the answer to a nickel allergyLight, highly corrosion-resistant, marks well. Costs more and not every shop stocks it.
304 stainless — fineA little behind 316L against sweat, cheap and everywhere. A perfectly reasonable choice.
430 stainless — the weak oneDespite being what a lot of blank tags are made from. Least corrosion-resistant of the three. That matters here for a reason that is not cosmetic: a pitted or tea-stained surface loses the contrast the code depends on. Acceptable for a pendant worn over clothing; the poorest choice against the wrist all day.

What to avoid, and why

Sterling silver, brass, bronzeThey tarnish, and tarnish is exactly the loss of contrast that stops a code scanning. A piece that works the week you get it can quietly stop working a year later with nothing visibly wrong.
Anything plated or coatedGold-plated, black-coated, anodized. Two separate problems. The laser usually cuts through the coating to the metal underneath, so the mark is not the color you expected. And the coating wears: if the code is made of coating, wear destroys the code gradually and invisibly.
AluminumIt marks with good contrast but is soft, and ordinary wear scratches it. Scratches across a code spend error-correction margin you do not have much of.

About nickel

Stainless steels contain nickel — 316L rather more than 304 — but in solid stainless it is bound into the alloy and very little reaches skin, which is why 316L is the body-jewelry standard despite containing more of it. Plated metals are the usual cause of reactions, which is a second reason to avoid them. If you have a diagnosed nickel allergy, titanium avoids the question altogether.

Finish and process

Matte, and cut by a fiber laser

Matte, bead-blasted, brushed or tumbledAny of these are fine.
Not polished, and never mirrorA shiny surface throws the phone's own light straight back into the camera. It is the most common reason an engraved code will not scan, and it cannot be fixed afterwards.

The jewelry trade sells on shine, so matte usually has to be asked for by name.

Fiber laserEach module is a solid filled square about half a millimeter across, and the machine has to hold that cleanly.
Rotary and diamond-drag engraving cannot do thisA stylus tracing lines cannot fill squares that small. Shops that only offer these can engrave your name beautifully and still not be able to do this.
Ask for a dark markLaser annealing or deep etching. A faint gray scratch on bright steel is the weakest result and the likeliest to fail.

Mistakes

Things that quietly ruin it

These are the mistakes a competent, well-meaning engraver makes, and each one is worth saying out loud when you place the order.

Do not resize it to fit the plate“Fit to page” and “optimize for the blank” change the module size silently, and the plate is almost never the size the symbol needs.
Do not crop the clear marginThe blank border is part of the symbol. Without it many phones will not detect that a code is there at all.
Do not add a logo, a border, or rounded corners to the modulesStyling that looks fine on screen costs real scans on metal.
Do not retype the address to regenerate the codeA retyped address produces a different symbol, and a symbol pointing somewhere else is worse than no symbol.

Have it tested before you accept it

Ask the engraver to scan the finished piece with a phone camera, twice:

In dim indoor light, at arm's length — about 40 cm.
Under a bright light at a shallow angle.

It has to resolve first time in both. The second test is the one that finds a reflective finish, and a piece that passes only the first will fail in a lit clinical room — which is exactly where it will be used.