UHF Read-Range Estimator
Friis-equation free-space range for any passive UHF tag chip, plus a realistic 40-70% band. Region toggle handles the FCC vs ETSI conversion for you.
Typical passive UHF dipole ≈ 2 dBi. Small labels 0-1 dBi; enterprise ATID/tags up to 4 dBi.
Formula, worked with your inputs
- Wavelength λ = c / f = 3×10⁸ / (915×10⁶) = 0.3277 m
- Reader EIRP = 36 dBm → Pt·Gt = 3981.07 mW
- Tag sensitivity = -24 dBm → required Pr = 3.981 μW
- Tag antenna gain = 2 dBi → Gr = 1.58
- d = (λ / 4π) · √(Pt·Gt·Gr / Pr) = (0.0261) · √(1.585e+6) = 32.82 m (free-space theoretical)
About Friis and this estimator
Passive UHF RFID range is set by two things: how loudly the reader transmits, and how faintly the tag chip can still wake up. The Friis transmission equation ties them together: the power arriving at the tag falls with the square of distance, so doubling range needs four times the transmit power. In the US, FCC Part 15 lets a reader radiate up to 36 dBm EIRP (4 W); ETSI in Europe caps at 33 dBm ERP, which is 35.15 dBm EIRP once you add the 2.15 dB dipole reference — a small but real difference in maximum theoretical range. Tag chip sensitivity is the other lever. A modern chip like NXP UCODE 9 at -24 dBm can wake up on roughly 4 microwatts; older parts like Alien Higgs-3 need -18 dBm (16 microwatts) — four times as much power for the same reader, which cuts theoretical range in half. Antenna gain matters on both sides, but for a passive tag it is usually 1-2 dBi. This calculator returns free-space range from Friis directly, then multiplies by 0.4-0.7 to bracket what you'll actually measure — reflections, absorption in water and cardboard, tag orientation, and label detuning against metal or liquid all cut real-world range well below the free-space number. Use free-space as a ceiling; use the realistic band when quoting a project.
FAQ
Why is the realistic range so much lower than Friis?
Friis assumes free space. Any real environment — reflectors, absorbers, cross-polarised tag orientation, humid air, cardboard, human bodies — subtracts several dB of link budget. 40-70% of theoretical is the range most integrators actually measure.
What's the difference between EIRP and ERP?
EIRP is referenced to an isotropic radiator; ERP is referenced to a half-wave dipole. ERP + 2.15 dB = EIRP. FCC quotes EIRP; ETSI EN 302 208 quotes ERP.
Why is 866 MHz range slightly longer than 915 MHz?
Lower frequency → longer wavelength → less free-space path loss for the same distance. The Friis formula has λ² in the numerator.
Which chip sensitivity should I use if I can't find the datasheet?
Use the manual entry field. Typical modern UHF chips are between -20 and -24 dBm; sub-optimal or budget parts sit around -15 to -18 dBm.