It depends on the type of passive RFID tag. Passive LF and HF tags have a read range of up to 90cm, while a simple passive UHF tag can be read from 4m-6m in free air (that is, not put the tag on an object). 

Over the past few years, designers of passive UHF tags have been able to develop special tags that can be read from farther away. 

(1)There are passive tags, such as our micro rfid tag OPPD5, size D5mm, which have the tag’s antenna built into the tiny tag. Although the antenna is so small, still get 1m read distance.
Lots of our mini rfid tags reach more than 1m read distance.

(2)Our long range rfid tag OPP130 are designed for larger assets, being read from a distance of 30m. 

(3)As we know, read distance depends on both tag and reader. So UHF systems designers are also working on readers to increase read range. 
Separate the reader’s send and receive function, use a phased-array antenna, some reader can read normal passive UHF tags from 300 feet away.

So in short, passive tags can be read from 1 millimeter to more than 300 feet away.
We are specialized in rfid long range tag and rfid small tag. Welcome for inquiry.

Michelle Zheng
Tel: 8618750070819
Email: sales11@oppiot.com