dBm to dBi Calculator

dBi stands for deciBel Isotropic. It is the gain of an antenna relative to that of an ideal isotropic antenna that radiates energy equally in all directions.

dBm is the power relative to one milliwatt. dBm can be directly converted to Watt and vice versa.

This tool uses the

  • Power at the output of a transmitter (dBm)
  • Cable loss (dB) and
  • EIRP (dBm)

to find the required antenna gain in dBi.

dBi to dBm

Formula

GAnt = EIRP – PTx + LC

Example Application

The maximum EIRP for a LoRa device operating at 922 MHz in Japan is +27 dBm.

Assuming a cable loss of 1 dB and a maximum output power from the transmitter of +15 dBm, the maximum allowable antenna gain is 13 dBi.

dBm to dBi Table

The FCC specifies a max EIRP of +36 dBm for Wi-Fi systems. The table below shows the max antenna gain (dBi) for a given transmit power (dBm) with no cable loss.

Antenna Gain (dBi)Transmit Power (dBm)
56-20
55-19
54-18
53-17
52-16
51-15
50-14
49-13
48-12
47-11
46-10
45-9
44-8
43-7
42-6
41-5
40-4
39-3
38-2
37-1
360
351
342
333
324
315
306
297
288
279
2610
2511
2412
2313
2214
2115
2016
1917
1818
1719
1620
1521
1422
1323
1224
1125
1026
927
828
729
630
531
432
333
234
135
036
-137
-238
-339
-440
-541
-642
-743
-844
-945
-1046
-1147
-1248
-1349
-1450