A 100 mW radio output
10 log₁₀(100) = 20 dBm. This is the radio output power, not necessarily the EIRP after its cable loss and antenna gain.
Power level relative to 1 mW 20 dBm Open in the calculator
dBm to mW and watts converter
Enter a power in any one of the three units. Negative dBm values are valid: −30 dBm is one microwatt.
Power level relative to 1 mW
20dBm
Check the values to see a result.
- Milliwatts
- 100 mW
- Watts
- 0.1 W
Negative dBm means less than 1 mW, not negative physical power. A dB gain or loss is a ratio, not an absolute power.
How it’s calculated
- PdBm = 10 log₁₀(PmW)=10 log₁₀(100)=20 dBm
- PmW = 10^(PdBm / 10)=10^(20 / 10)=100 mW
- PW = PmW / 1,000=100 / 1,000=0.1 W
dBm = 10 log₁₀(mW)mW = 10^(dBm / 10)W = mW / 1,000
20 dBm = 100 mW = 0.1 W.
Tip: A 10 dB increase multiplies power by ten. A 3 dB increase is approximately a doubling.
Convert absolute power, not a ratio
dBm expresses power relative to one milliwatt. Zero dBm is 1 mW, 20 dBm is 100 mW and 30 dBm is 1 W. A negative value represents a positive power below 1 mW: −30 dBm is 0.001 mW, or 0.000001 W.
From dBm
mW = 10^(dBm / 10)
Divide the dBm value by ten and use it as the exponent of ten. The result is in milliwatts. Divide by 1,000 to obtain watts.
To dBm
dBm = 10 log₁₀(mW)
First convert watts to milliwatts if needed. The logarithm takes a positive power relative to 1 mW. Zero watts has no finite dBm value and negative physical power is outside this conversion.
Power gain and loss
Power ratio = 10^(dB / 10)
A 10 dB gain multiplies power by ten. A 3 dB gain multiplies it by about 1.995, close to two. A 3 dB loss divides by the same factor. These are relative changes; dB alone does not identify an absolute power.
dBm, milliwatts and watts examples
A −90 dBm received signal
A -90 dBm signal: 10^(−90 / 10) = 0.000000001 mW, equal to 0.000000000001 W, or one picowatt. Negative dBm does not mean a negative number of watts.
Power level relative to 1 mW -90 dBm Open in the calculator
Questions about dBm, milliwatts and watts
Is 0 dBm no signal?
No. It is exactly 1 mW. Zero watts approaches negative infinity dBm and has no finite result in this calculator.
Can I add two dBm power values?
Convert each to watts or milliwatts, add the powers where that physical model applies, then convert the total back. Two equal independent 1 mW powers total 2 mW, approximately 3.0103 dBm. Phase-coherent signals require their own treatment.
Why is there no voltage conversion?
A voltage needs an impedance and an RMS convention before it determines power. This tool converts power units only. For power ratios use 10 log₁₀; the familiar 20 log₁₀ voltage ratio applies under the appropriate equal-impedance assumptions.
Limits of this result
- This converts absolute power only. It does not convert field strength, antenna gain, RSSI quality or a legal transmit limit.
- Enter a positive linear power. Zero power corresponds to negative infinity dBm and is outside this finite-number calculator.