A dBm is a unit of power expressed using a logarithmic scale with respect to one milliwatt. It is sometimes seen as dBmW for clarity. Power in watts is a linear unit, i.e., 20 Watts is two times a 10-Watt level and ½ a 40 Watt level. However, 20 dBm is 10 times as much as 10 dBm and 100 times less than 40 dBm.
Here is a table of some dBm values and their Watt equivalence. Notice higher dBm values are simpler and will be easier to work with.
Power (W) | Power (dBm) | Power (W) | Power (dBm) |
0.000001 | -30.0000000 | 5 | 36.9897000 |
0.00001 | -20.0000000 | 6 | 37.7815125 |
0.0001 | -10.0000000 | 7 | 38.4509804 |
0.001 | 0.0000000 | 8 | 39.0308999 |
0.01 | 10.0000000 | 9 | 39.5424251 |
0.1 | 20.0000000 | 10 | 40.0000000 |
1 | 30.0000000 | 20 | 43.0103000 |
2 | 33.0103000 | 40 | 46.0205999 |
3 | 34.7712125 | 100 | 50.0000000 |
4 | 36.0205999 | 500 | 56.9897000 |
Also note each 10 dBm increase results in a multiple of 10 increase in Watts. 30 dBm is 1 Watt, and 40 dBm is 10 Watts.
Now, let’s go the other way from Watts to dBm and see what a table shows. See the table on the next page.
Formulas for calculating Watts from dBm are shown below.
PW = 1W x 10^(PdBm / 10) / 1000 or
PW = 10^((PdBm - 30) / 10) A little more complex due to parentheses.
Power (W) | Power (dBm) | Power (W) | Power (dBm) |
0.000001 | -30.0000000 | 5 | 36.9897000 |
0.00001 | -20.0000000 | 6 | 37.7815125 |
0.0001 | -10.0000000 | 7 | 38.4509804 |
0.001 | 0.0000000 | 8 | 39.0308999 |
0.01 | 10.0000000 | 9 | 39.5424251 |
0.1 | 20.0000000 | 10 | 40.0000000 |
1 | 30.0000000 | 20 | 43.0103000 |
2 | 33.0103000 | 40 | 46.0205999 |
3 | 34.7712125 | 100 | 50.0000000 |
4 | 36.0205999 | 500 | 56.9897000 |
A couple items of interest we can see in the table. If we double the power in Watts, the increase in dBm is approximately 3. One Watt is 30 dBm, and 2 Watts is 33.0103 dBm. If we divide the power in half, the dBm value decreases by approximately 3. Twenty Watts is 43.0103 dBm, and 10 Watts is 40 dBm. Another interesting fact is that 1 milliWatt is zero dBm. Zero dBM is the reference to a milliWatt.
Many calculations become easier when using dBm rather than Watts, but that topic is beyond the scope of this document.
Formulas for calculating dBm from Watts are shown below.
PdBm = 10 x log10(1000 x PW / 1W) or
PdBm = 10 x log10(PW / 1W) + 30
If you are interested in a somewhat prettier document, please visit my Google Drive here.
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