Can you cut an NFC tag to size?
No. The antenna is a coil filling nearly the whole tag, so a cut almost anywhere severs the loop. A severed loop collects no energy, which means the tag is dead rather than weakened. There is no repair, no partial function, and no way to tell by looking.
Why a cut is fatal
The antenna is a continuous conductor wound in several turns, and it occupies almost the entire area of the tag. There is very little tag that is not antenna.
The chip draws its power from the current induced around that closed loop. Break the loop anywhere and no current flows, so the chip never wakes up.
This is not a degradation. There is no shorter range, no intermittent reading, no working at close contact. The tag is inert.
And a trimmed tag looks entirely normal, because the break is a hairline in a conductor under the surface. Nothing warns the person who cut it.
What you can trim
Some tags carry a printed margin marking material outside the antenna, and trimming within that margin is safe. If the tag does not carry such a marking, assume there is no margin.
Backing paper and liner can be cut freely, since they are not part of the tag.
Rounding a corner on a card whose inlay is centred is sometimes tolerable, and it is a gamble unless the supplier states where the antenna reaches. Ask rather than guess.
Everything else is a cut through the antenna waiting to happen. Punching a hole for a keyring is a cut, and it kills a tag as thoroughly as trimming an edge.
What to do instead
Buy the size you need. Tags exist from a few millimetres upward, and the small ones cost no more than the large ones.
Accept that a smaller tag reads at a shorter distance, since the antenna is smaller. This is the real trade off, and it is the reason people try cutting in the first place: they want the range of a large tag in the footprint of a small one, and physics does not offer that.
Where space is genuinely tight, consider a different form factor rather than a smaller version of the same one. An inlay embedded in the object can use space a stuck on tag cannot.
If you have already cut one
Test it before assuming the worst, because a lucky cut outside the coil leaves the tag working.
If it does not read on any phone, it is not recoverable. The conductor is thin, the geometry is tuned, and a repair that restores continuity changes the tuning.
Do not repeat the test on the rest of the batch. One cut tag is a lesson and a hundred is a write off.
Frequently asked questions
Can I punch a hole in a tag for a keyring?
Not unless the tag was made with one. A punched hole cuts the antenna in most tags, and keyfobs exist precisely because this is not something you can add.
How do I know where the antenna reaches?
Ask the supplier, since it is not visible from outside on most products. Some tags are printed with a safe trim margin, and where there is none, assume the coil reaches the edge.
My cut tag still reads. Did I get away with it?
Probably, and the tag may now be more fragile at the cut edge. Verify it, and do not treat one success as permission for the batch.
Can a cut tag be repaired?
No, in practice. Restoring continuity in a thin tuned conductor is not something that can be done reliably, and a replacement tag costs less than trying.