The Approximation Process
Now we are ready to convert from dBm to power levels.
The first step is to see how many factors of 30 are in the number you are trying to convert.
If the number is greater than or equal to 30, subtract 30 from the dBm value you are trying to convert repeatedly without allowing the number to go negative. Each factor of 30 you subtract moves you up the power scale from milliwatts to Watts, kilowatts, or megawatts.
For example, to express 10,000 milliwatts in terms of watts, you would divide 10,000 milliwatts by a factor of 1,000 milliwatts/1 watt to yield the final answer of 10 watts. Remember that dividing by 1,000 is the same as subtracting 30dB in the dB system.
If the number is negative, add 30 to the number until it is positive. Each factor of 30 you have to add moves you down the scale from microwatts to nanowatts to picowatts, etc.
Example: Suppose you want to express 77 dBm in terms of watts or kilowatts. We subtract 30 from 77, which moves us from milliwatts to watts. The result is 47 dB worth of watts. We can subtract another 30 from this without going negative, so we do and move up the scale from watts to kilowatts. Now the result is 17 dB worth of kilowatts.
Now all we have to do is figure out what 17 dB worth of kilowatts actually is.
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