Decibel converter: dBm, mW, W, dBW and dB ratios
Our decibel converter links absolute power (dBm, dBW, W, mW, uW) and dB ratios on one surface. Power uses 10 log10, voltage uses 20 log10 (because P is proportional to V squared). Landmarks: 0 dBm = 1 mW, +30 dBm = 1 W, +3 dB doubles power.
dB to ratio: power 10 log, voltage 20 log
0 dBm
-30 dBW · 1 mW (1 mW)
Base-10 decibels. 0 dBm = 1 mW. Power ratios use 10 log, voltage ratios 20 log; a voltage-to-power crossing needs the reference impedance.
FAQ
- Why does power use 10 log and voltage use 20 log?
- Because power is proportional to voltage squared at equal impedance (P = V squared / Z). A voltage ratio r therefore gives a power ratio r squared, and 10 log10(r squared) = 20 log10(r). The factor 20 is not a separate rule, it is the factor 10 applied to a quadratic quantity. Reference: IEC 60027-3.
- What is the difference between dBm and dBW?
- dBm references 1 milliwatt, dBW references 1 watt. Since 1 W = 1000 mW and 10 log10(1000) = 30, you always have dBm = dBW + 30. So 0 dBm = -30 dBW = 1 mW, and +30 dBm = 0 dBW = 1 W. Our tool shows both references plus the linear values (W, mW, uW) side by side.
- What do the +3 dB, +6 dB and -3 dB landmarks mean?
- +3 dB is a doubling of power (exactly +3.01 dB for x2). +6 dB is a doubling of voltage (exactly +6.02 dB), which is also x4 in power. -3 dB is half power, the -3 dB point that defines a filter bandwidth. These landmarks let you sanity-check a calculation in your head before trusting the tool.
- Why is a reference impedance needed to go from voltage to power?
- A voltage alone does not define a power: you need P = V squared / Z. Without an impedance, the voltage to absolute power crossing is undefined. We offer 50 ohm (RF, default), 75 ohm (video, antenna) and 600 ohm (audio). For a pure ratio at one constant impedance, Z cancels out and is not required.
- Why is a ratio of 0 or negative rejected?
- The base-10 logarithm is only defined for strictly positive values. A ratio of 0 would return minus infinity, and a negative ratio has no physical meaning for a power or an amplitude. The tool therefore requires a ratio greater than zero and returns a message rather than a NaN or infinite result, keeping every conversion usable.
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AESTECHNO is an electronics design office near Montpellier, France, with 10+ years of experience in RF, IoT and embedded design, and a 100% first-pass rate on CE/FCC certification.