The dB measures the power of a signal as a function of its ratio to another standardized value. The abbreviation dB is often combined with other abbreviations in order to represent the values that are compared. Here are two examples:

dBm—The dB value is compared to 1 mW.

dBw—The dB value is compared to 1 W.
You can calculate the power in dBs from this formula:
Power (in dB) = 10 * log10 (Signal/Reference)
This list defines the terms in the formula:

log10 is logarithm base 10.

Signal is the power of the signal (for example, 50 mW).

Reference is the reference power (for example, 1 mW).
Here is an example. If you want to calculate the power in dB of 50 mW, apply the formula in order to get:
Power (in dB) = 10 * log10 (50/1) = 10 * log10 (50) = 10 * 1.7 = 17 dBm
Because decibels are ratios that compare two power levels, you can use simple math in order to manipulate the ratios for the design and assembly of networks. For example, you can apply this basic rule in order to calculate logarithms of large numbers:
log10 (A*B) = log10(A) + log10(B)
If you use the formula above, you can calculate the power of 50 mW in dBs in this way:
Power (in dB) = 10 * log10 (50) = 10 * log10 (5 * 10) = (10 * log10 (5)) +
(10 * log10(10)) = 7 + 10 = 17 dBm
These are commonly used general rules:
An Increase of: 
A Decrease of: 
Produces: 
3 dB 
Double transmit power 

3 dB 
Half transmit power 

10 dB 
10 times the transmit power 

10 dB 
Divides transmit power by 10 

30 dB 
1000 times the transmit power 

30 dB 
Decreases transmit power 1000 times 
This table provides approximate dBm to mW values:
dBm 
mW 
0 
1 
1 
1.25 
2 
1.56 
3 
2 
4 
2.5 
5 
3.12 
6 
4 
7 
5 
8 
6.25 
9 
8 
10 
10 
11 
12.5 
12 
16 
13 
20 
14 
25 
15 
32 
16 
40 
17 
50 
18 
64 
19 
80 
20 
100 
21 
128 
22 
160 
23 
200 
24 
256 
25 
320 
26 
400 
27 
512 
28 
640 
29 
800 
30 
1000 or 1 W 
Here is an example:

If 0 dB = 1 mW, then 14 dB = 25 mW.

If 0 dB = 1 mW, then 10 dB = 10 mW, and 20 dB = 100 mW.

Subtract 3 dB from 100 mW in order to drop the power by half (17 dB = 50 mW). Then, subtract 3 dB again in order to drop the power by 50 percent again (14 dB = 25 mW).
Note: You can find all values with a little addition or subtraction if you use the basic rules of algorithms.
Antennas
You can also use the dB abbreviation in order to describe the power level rating of antennas:

dBi—For use with isotropic antennas.
Note: Isotropic antennas are theoretical antennas that transmit equal power density in all directions. They are used only as theoretical (mathematical) references. They do not exist in the real world.

dBd—With reference to dipole antennas.
Isotropic antenna power is the ideal measurement to which antennas
are compared. (dBi)
Dipole antennas are realworld antennas, dometimes antennas are rated
in dBd
The power rating difference between dBd and dBi is approximately 2.2—that is, 0 dBd = 2.2 dBi. Therefore, an antenna that is rated at 3 dBd is rated as 5.2 dBi.
Effective Isotropic Radiated Power
The radiated (transmitted) power is rated in either dBm or W. Power that comes off an antenna is measured as effective isotropic radiated power (EIRP). EIRP is the value that regulatory agencies, such as the FCC or European Telecommunications Standards Institute (ETSI), use to determine and measure power limits in applications such as 2.4GHz or 5GHz wireless equipment. In order to calculate EIRP, add the transmitter power (in dBm) to the antenna gain (in dBi) and subtract any cable losses (in dB).
Path Loss
The distance that a signal can be transmitted depends on several factors. The primary hardware factors that are involved are:

Transmitter power

Cable losses between the transmitter and its antenna

Antenna gain of the transmitter

Localization of the two antennas
This refers to how far apart the antennas are and if there are obstacles between them. Antennas that can see each other without any obstacles between them are in line of sight.

Receiving antenna gain

Cable losses between the receiver and its antenna

Receiver sensitivity
Receiver sensitivity is defined as the minimum signal power level (in dBm or mW) that is necessary for the receiver to accurately decode a given signal. Because dBm is compared to 0 mW, 0 dBm is a relative point, much like 0 degrees is in temperature measurement. This table shows example values of receiver sensitivity:
dBm 
mW 
10 
10 
3 
2 
0 
1 
3 
0.5 
10 
0.1 
20 
0.01 
30 
0.001 
40 
0.0001 
50 
0.00001 
60 
0.000001 
70 
0.0000001 
The receiver sensitivity of the radios in Aironet products is 84 dBm or 0.000000004 mW.