Does the reader or the tag determine the range?
Both ends contribute, and in practice the tag is the one worth changing. Reader output is capped by regulation and fixed by the hardware you already own, while tag antenna size is a specification you control at the moment of purchase for a few cents.
Both, but not equally
Range comes from the coupling between two coils, so both matter physically. The useful question is not which contributes more, it is which one you can actually change.
That question has a clear answer, and it is the tag.
What the reader side offers
Very little room to manoeuvre when the reader is a phone. The antenna size, its position and the field it produces are fixed by the manufacturer. Regulation caps how much field any reader may generate, so nobody is holding back a more powerful mode.
Phones do differ from each other, and the differences are real but modest and unpredictable. They do not track price or how recent the handset is, so choosing a phone for its NFC range is not a strategy.
Where the reader side genuinely opens up is when you control it. A fixed reader with a large antenna reads far further than a phone, which is why library gates and access barriers achieve distances a phone never will.
What the tag side offers
A specification you set before buying, for a few cents. Antenna size is the dominant variable, and moving from a small sticker to a card sized tag is the most effective single change available.
What the tag also offers is the ability to be wrong for its environment. A tag on metal, a tag creased during application, a tag stacked with another: all of these are tag side problems with tag side solutions, and none of them are helped by a different phone.
The order to try things in
When a read is too short or unreliable, work through this sequence and stop when it works.
First, move the phone around to find the antenna, since a surprising share of complaints are aim rather than range. Second, remove the case and any card in it. Third, take the tag off whatever it is stuck to and read it in the air, which tells you immediately whether the environment is the problem. Fourth, try a larger tag.
Only after all four does buying different hardware make sense, and by then you usually know why.
The one situation that reverses the priority is a deployment where you supply the reader. There, specifying a proper fixed reader with a large antenna solves problems no tag can, and it is worth considering before compromising on tag size.
Frequently asked questions
Can I buy a more powerful phone for NFC?
No handset is marketed on NFC range, and the variation between models is small and inconsistent. If range is the requirement, a dedicated reader is the answer rather than a different phone.
Do phone cases explain most range complaints?
Cases and aim together explain a large share of them. Both are free to rule out, which is why they come first in any diagnosis.
Does a stronger reader compensate for a small tag?
Partly, and there are limits. A large fixed reader will read a small tag further than a phone does, and it cannot rescue a tag that is detuned by its surroundings.
Is there a maximum range I should promise?
Promise a tap rather than a distance. Any figure you quote will be measured by someone with a different phone, a thicker case and a tag stuck to something you did not anticipate.