This is a math/engineering lesson. Save yourself!!
While trying to get some info on the output power of various Linksys wireless systems, I have found all sorts of misinformation and/or confusion regarding dBm and milliwatts (mW). Some have incorrectly said that 0dBm=0mW. Some have said that the WRT54G outputs 84dBm, which is actually 250 Kilowatts!!!
Yeah, I know, we all (incl. me) make stupid mistakes and occasionally use the wrong terminators. Here is a definitive description of the two terms.
0 dBm in the RF world is arbitrarily defined as 1 mW of power. From this, we get:
dBm = 10 * log(mW) (Note: power is in mW, not Watts!)
mW = 10 ^ (dBm/10)
Both of these can easily be calculated using the Windows Calculator in Scientific mode.
Here is a simple cross reference, including some commonly found values for wireless. Remember, for all practical purposes, every 3 dB is a doubling/halving of power, and every 10 dB is an order of magnitude difference.
-10 dBm = 0.1 mW
0 dBm = 1.0 mW
10 dBm = 10 mW
17 dBm = 50 mW
19 dBm = 79 mW
20 dBm = 100 mW
23 dBm = 200 mW
26 dBm = 400 mW
27 dBm = 500 mW
28 dBm = 631 mW
30 dBm = 1 Watt
40 dBm = 10 Watts
50 dBm = 100 Watts
60 dBm = 1000 Watts ...etc.
Some of the above numbers are approximated for clarity; 26 dBm actually equals 398.107 mW.
Use the term "dBm" when referring to some specific power level; never just "dB." e.g. "This router has 23 dBm of power."
The phase, "This outputs 25 dB" is technically meaningless, although most will interpret this to mean 25 dBm.
The term "dB" is a relative term only; such as, "This router has 5 dB more power than that AP"
Let's keep those terminators straight!