The Formula
P(W) = 10^(P(dBm)/10) / 1000
dBm expresses absolute power relative to 1 milliwatt. A negative dBm value is a positive power below 1 mW; it is not negative watts. Device transmit limits and receiver sensitivity depend on the equipment and operating mode, not on this unit conversion.
dBm to watts and milliwatts conversion table
| dBm | Watts (W) | Milliwatts (mW) |
|---|---|---|
| -100 | 1e-13 | 1e-10 |
| -60 | 1e-9 | 0.000001 |
| -30 | 0.000001 | 0.001 |
| -10 | 0.0001 | 0.1 |
| 0 | 0.001 | 1 |
| 3 | 0.00199526 | 1.99526 |
| 10 | 0.01 | 10 |
| 20 | 0.1 | 100 |
| 23 | 0.199526 | 199.526 |
| 30 | 1 | 1000 |
| 40 | 10 | 10000 |
| 50 | 100 | 100000 |
Convert watts or mW back to dBm
Use the same converter above in the reverse direction: choose Watts to dBm, then enter a positive watt value. The relationship is P(dBm) = 10 log10(P(W) × 1000), or 10 log10(P(mW)) when the input is in milliwatts. Zero watts has no finite dBm value.
For example, 30 dBm is 1 W or 1000 mW; 100 W is 50 dBm; and −30 dBm is 0.000001 W or 0.001 mW. A 10 dB increase multiplies power by ten. A 3 dB increase is approximately, not exactly, a doubling.
dBm versus dB in a link budget
dBm is an absolute power; dB describes a ratio, gain or loss. Subtract a 3 dB cable loss from 30 dBm to get 27 dBm, about 0.501 W. Use the coax cable loss calculator for the feeder and the RF link budget calculator to combine gains, losses and receiver sensitivity.