mW to dBm Calculator

Convert transmit power and signal levels between milliwatts and dBm, see the formula with your numbers, and look up common values in the table.

Convert

Type in any box and the others update. dBm can be negative: received Wi-Fi signals are usually well below 0 dBm.

The formula and your numbers

mW to dBmdBm = 10 × log₁₀(P in mW)
dBm to mWP in mW = 10dBm ÷ 10

dBm to mW conversion table

Click a row to load it into the converter. Values are rounded; the converter shows them exactly.

+3 dB ≈ twice the power100.3 = 1.995, so 3 dB more is almost exactly double: 17 dBm ≈ 50 mW, 20 dBm = 100 mW.
+10 dB = 10 × the power101 = 10 exactly: 0 dBm = 1 mW, 10 dBm = 10 mW, 20 dBm = 100 mW, 30 dBm = 1 W.
−3 dB ≈ half the powerHalving the power always takes away about 3 dB, whatever the starting level.
0 dBm = 1 mWdBm is decibels compared with 1 milliwatt, so 1 mW is the zero point and anything below it is negative.

Notes

Why Wi-Fi uses dBm

Wi-Fi powers cover a huge range: an access point may transmit 100 mW (20 dBm) while a client far away receives a millionth of a millionth of that. On a decibel scale those become manageable numbers, and gains and losses simply add and subtract: 20 dBm transmitted minus 70 dB of path loss is −50 dBm received.

dBm, dB and dBi

dBm is an absolute power level compared with 1 mW. dB on its own is a ratio between two powers, such as a gain or a loss. dBi is an antenna's gain compared with an ideal antenna that radiates equally in every direction. That's why you add dB and dBi to dBm, but never add two dBm values together.

Adding two signals

To combine two powers, convert them to mW first, add, then convert back. Two 20 dBm signals (100 mW each) make 200 mW, which is 23 dBm, not 40 dBm.

Related tools

See how dBm levels fall with distance in the Free Space Path Loss tool, and why the received level matters in the Wi-Fi Modulation Simulator.