# Forum

## dbm to watts-- p309 version 3 cwna study guide

3 posts by 3 authors in: Forums > CWNA - Enterprise Wi-Fi Admin
Last Post: February 9, 2006:
• On the figure 8.3 point-to-point limit table, I got dbm to watts all entries correct except 3. please tell me what I did wrong.
1. EIRP = 46dbm Book says it equal to 39.8 watts, but I got 40watts.
my calculation:
46 = 40+6dbm
40 is 10watts
dbm means double then double, 4*10 = 40 watts
2. EIRP = 48dbm Book says it equal to 63 watts, but I got 62.5 watts.
My calculation:
48=60-12
60 is 1000 watts
-12 means half, half, half then half
1000/16 =62.5 watts
3. EIRP = 52 dbm Book says it equal to 158 watts, but I got 160 watts.
My calculation:
53=40+12
40 is 10 watts
so it will be 10*16=160 watts.

• Hi Phelan:

The rules of 10's and 3's are approximations. Other than 10 and 100, the values for watts in figure 8.3 are also approximations.

In my experience with CWNP Program exams, the rules of 10's and 3's led directly to the correct multiple choice answer.

In my experience with Cisco WLAN exams, the rules of 10's and 3's led close enough to the correct multiple choice answer to get the job done.