K4OZI / Part 15 Power
Maximum legal transmit power for LoRa in the US ISM bands

Not legal advice and not authoritative. This page is an engineering estimator. Verify every result and every rule against the current text of 47 CFR Part 15, the FCC KDB guidance that applies to your device, and measurements from an accredited lab. Rules change, this calculator models them imperfectly, and only a TCB can authorise equipment.

How far under the limit line SF7 at 125 kHz lands.

Every spreading factor at the selected bandwidth, against the 1 watt conducted ceiling in 15.247(b). Hover a bar for the detail.

Maximum conducted power by spreading factor A bar chart of the highest legal conducted output power at each spreading factor, with the one watt regulatory limit drawn as a dashed line.
Selected spreading factor Other spreading factors Your planned power Density ceiling, 15.247(e)/(f) 1 W ceiling, 15.247(b) Falls back to 15.249

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Preliminary estimate, not legal advice — verify the rules and confirm on a bench before you transmit.

Planned at antenna
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Margin
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Your EIRP
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Maximum conducted power
30.0dBm
1000 mW
Maximum EIRP
32.0dBm
1585 mW
Limited by
hopping channel count
15.247(a)(1)(iii), 15.247(b)(3)

Radio

What the radio puts out at its own RF port.

Antenna

At or below 6 dBi nothing is given back.

Only relaxes the gain penalty in the 2.4 GHz band.

Hopping

Transmission

Measurement assumptions

The 6 dB points sit below the −3 dB points, so this is never under 1.0. A measured 500 kHz LoRa channel read 697 kHz. Decides whether 15.247(a)(2) is met.

Sets the hopping channel minimum, the channel separation, and whether the 500 kHz hopping cap is breached.

Average is exact for a chirp and does not vary with spreading factor. The peak figure models a analyser artefact, not the signal.

About 1 for a digital or FFT resolution filter, about 2 for a 4-pole analog one. Only affects the peak figure.

A register bit, and the ecosystem disagrees: Semtech's own stack enables it only at SF11–12/125 kHz and SF12/250 kHz, never at narrow bandwidths.

The four routes

A LoRa radio that hops may certify as frequency hopping, as digital modulation, or as a hybrid — the classification is the applicant's choice, not a consequence of hopping. This page scores all of them, plus the 15.249 floor, and takes the one that legally allows the most power.

Power spectral density

8 dBm in any 3 kHz slice, from 15.247(e) on the digital route and from 15.247(f) on the hopping route. A flat reading assumes the energy spreads evenly; in reality the chirp sweeps, and a slow sweep parks more energy in each slice than a fast one.

Model Duty in 3 kHz Allowed power

Airtime and dwell

Under 15.247(f) a hybrid system may not hold one channel longer than 400 ms. This is the reason US915 has no SF11 or SF12 at 125 kHz.

Every spreading factor

The same sweep drawn above, as numbers.

SF Symbol time Airtime Conducted EIRP Route