How this calculator works
The decibel is a logarithmic ratio, and a suffixed decibel fixes the reference. dBm means decibels referenced to one milliwatt, so 0 dBm is exactly 1 mW and 30 dBm is 1 W. Every increase of 10 dB multiplies power by ten, and every increase of 3 dB roughly doubles it. Because the scale is logarithmic, negative values describe powers below 1 mW, such as the tiny signals at the end of a long fibre or radio link.
Formula
P(mW) = 10^(dBm/10). P(W) = P(mW)/1000. dBm = 10 log10(P(mW)) = 10 log10(P(W) x 1000).
Worked example
0 dBm is 1 mW or 0.001 W. 30 dBm is 1,000 mW or 1 W. -10 dBm is 0.1 mW or 0.0001 W. 1 W is 30 dBm, and 0.5 W is 26.9897 dBm.
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Frequently asked questions
What does the m in dBm mean?
Milliwatt. A dBm value is ten times the base-10 logarithm of the power divided by 1 mW, so the reference is built into the unit name.
Why is 30 dBm equal to 1 W?
1 W is 1,000 mW. The logarithm base 10 of 1,000 is 3, and ten times 3 is 30 dB. Therefore 1 W is 30 dBm.
Can dBm be negative?
Yes. A negative dBm value means less than 1 mW. For example, -30 dBm is 0.001 mW, or one microwatt.
Is dBm the same as dBW?
No. dBW is referenced to one watt, so a power in dBW is 30 dB lower than the same power in dBm. 0 dBW is 30 dBm.