Free tool
Passive RFID range calculator
How far will a passive UHF tag read? Pick a tag chip and a reader, and see the range in free space and the range you should plan for.
Work out a range
Range in free space
- m
These are ideal values. Real-life ranges are usually shorter, often by 30-40%.
- Radiated power (EIRP)
- Tag needs, at its chip
- Power link budget
- Reader needs, at its input
- Reply link budget
- Free-space loss at your distance
How it works
What it works out
A passive tag has no battery. The reader’s signal has to carry enough power to the tag to wake its chip, and then the tag’s reply has to be strong enough for the reader to hear. Either one can run out first, so the calculator does both sums and gives you the shorter distance.
The power link. Radiated power from the reader, plus the tag antenna’s gain, less the loss over the gap, has to reach the chip’s sensitivity. For most tags this is the one that limits.
The reply link. The tag reflects back only part of what reaches it. That reply crosses the gap again and arrives at the reader, which needs it to be above what its receiver can decode. Cheap readers run out here first. That’s the reason the reader chip is in the list.
What it leaves out
It assumes free space: no floor, no walls, no stock, no people, and a tag lined up perfectly with the antenna. Every number here is an ideal one, and real-life ranges are usually shorter, often by 30-40%. A real room is worse in places and occasionally better, because reflections add as well as cancel. That’s what the safety margin is for. Start at 6 dB. Go to 10 or more if there’s metal or liquid about, or if tags will be tilted.
It also takes the tag’s antenna gain on trust. A tag on a cardboard box and the same tag on a steel crate are different tags. If you know the number, type it in, and use a negative one for a bad surface.
Where the numbers come from
Tag chip sensitivities are the makers’ own figures, taken as the power the chip needs at its pins:
| Chip | Read | Write | Source |
|---|---|---|---|
| NXP UCODE 9 | -24 dBm | -22 dBm | NXP fact sheet |
| NXP UCODE 8 | -23 dBm | -18 dBm | NXP datasheet (-22.9 and -17.8 dBm typical) |
| Impinj Monza R6 | -22 dBm | -17 dBm | Impinj chip series sheet, 2020 |
A lower number is a more sensitive chip. Each dB of sensitivity is worth about 12% more range on the power link. Reader sensitivities are Impinj’s for the E310 and E710 chips in each reader mode, measured at the chip with a strong reflection from the antenna. Your reader will do a little worse than the chip, because of the coupler and the cable. The loss fields in the form let you account for that.
The spreadsheet we published years ago did only the power link, with a single sensitivity number for the tag. This one adds the reply link and the chips people ask about now. It’s still free to use, copy and change.
Choosing a mode
Impinj’s reader chips trade speed for hearing. The fast modes read hundreds of tags a second and hear less. The slow ones hear about 15 dB further and read a few dozen. If your range is short of what you need and the reader is the limit, try a slower mode before you buy a bigger antenna.
For how to pick between tag types in the first place, see selecting a suitable RFID system. For the readers we ship, see RFID hardware.