Does Android read tags with the screen off?
Generally not. Tag dispatch normally requires the screen on and the device unlocked, and the exact behaviour varies between manufacturers and versions. A phone asleep in a pocket is not looking for tags, which is a deliberate design rather than a limitation.
The normal behaviour
A phone with its screen off is generally not dispatching tags. Waking and unlocking it is what makes a tap work.
This is deliberate. A device that acted on tags while asleep in a pocket would be triggered by every tag it passed, which is neither useful nor safe.
Payment is the exception people notice, and it is a different mode entirely: the phone is answering a terminal rather than reading a tag, through a path with its own rules.
Why it varies
Manufacturers customise Android, and the behaviour around locked and sleeping devices is one of the things they customise.
Version differences add to that, so a rule established on one device may not hold on another from the same maker a year later.
This is why a confident general statement is not available here. What is available is the test.
Battery saving modes add a further layer, since some devices restrict NFC under them and the restriction persists in a way that looks like a fault.
Testing it on your devices
Lock the phone, present a known good tag, and observe. Then wake it without unlocking and repeat. Then unlock and repeat.
Three results, three states, and a clear answer for that device.
Repeat on the oldest and cheapest phone in your target population, since that is where behaviour is least generous.
Record what you found. It is the kind of detail that is quick to establish and infuriating to re establish six months later.
Designing around it
Assume the tap is deliberate and the phone is awake. That covers both platforms and removes the question.
Say what to do on the tag itself. A short instruction turns a puzzled visitor into a successful one.
Do not build a flow that requires a locked phone to read a tag. It works on some devices and not others, which is the worst of both.
And for anything hands free, treat the reliability of the read as the design constraint rather than an implementation detail.
Frequently asked questions
Why does payment work with the screen off?
Payment uses card emulation, where the terminal produces the field and the phone answers. That path has its own rules and is not tag reading.
Can I enable screen off reading?
It is not generally a user setting, and behaviour varies by manufacturer. Testing the specific device is the reliable route.
Does the phone need to be unlocked?
Usually yes for tag dispatch. Some devices differ, which is why the three state test is worth running.
Is this the same on iPhone?
The details differ. Background reading is available from the iPhone XS and XR onwards, and it presents a notification rather than acting directly.