What factors affect the read range of passive tags?

There are many factors that affect the read range of passive UHF RFID tags, including the output of the reader antenna (which is often limited by government regulation), the size of the tag, the sensitivity of the chip in the tag, the type of reader antenna used (circular-polarized, linear-polarized, phased-array, etc.), the material the tag is on and the environmental conditions.

I have seen a large passive UHF tag (Omni-ID‘s Ultra tag) be read from 80 feet away using a standard passive UHF reader with a linear-polarized antenna. I have also seen an ordinary 8-inch dipole tag be read from a distance of 60 feet via a phased-array antenna. And I have seen such a dipole tag be read from 600 feet with a phased-array antenna, with an exciter near the tag and the send and receive functions separated.

I think it makes more sense to discuss the average read distance of fairly standard dipole tags (if there is such a thing). Typically, you can read such a tag from a distance of 25 feet in free air with no problem.