About the dBm to Watts Converter
This dBm to watts converter turns a power level in any of the four common units — dBm, dBW, watts or milliwatts — into all of the others at once. Type 20 dBm and you immediately see 100 mW, 0.1 W and −10 dBW, along with a plain-English version of the number using the right SI prefix (µW, nW, kW and so on).
It is built for radio amateurs, RF and wireless engineers, Wi-Fi and antenna installers, and students who keep switching between logarithmic and linear power. Transmitter ratings, receiver sensitivity, link budgets and spectrum analyser readings are all quoted in dBm, while amplifier and power supply specs are usually in watts.
The tool also gives the RMS voltage that the power develops across a load (50 Ω by default, the standard for RF coax and test gear; use 75 Ω for TV/cable systems or 600 Ω for legacy audio). dBm is always referenced to 1 milliwatt, so 0 dBm = 1 mW regardless of impedance; only the voltage depends on the load.
With the default inputs, the power in watts is 0.1 W. Change any value above to recalculate instantly.
How to use the dbm to watts converter
- 1Enter the power value you have.
- 2Choose its unit: dBm, dBW, watts or milliwatts.
- 3Optionally change the load impedance (50 Ω for most RF gear).
- 4Read the equivalent power in every unit and the RMS voltage across the load.
- 5Use the reference table to sanity-check typical signal levels.
Formula and method
dBm expresses power as a ratio to one milliwatt on a logarithmic scale: dBm = 10 × log10(P ÷ 1 mW). Reversing it gives P in milliwatts = 10^(dBm/10), and dividing by 1,000 converts to watts. dBW uses 1 watt as the reference instead, and because 1 W = 1,000 mW = +30 dB, dBW is always exactly 30 less than dBm.
The voltage result comes from P = V² ÷ Z for a purely resistive load, so Vrms = √(P × Z). For a sine wave the peak voltage is Vrms × √2. Zero or negative linear power has no decibel value, so the tool shows 0 and a note instead of minus infinity.
- P
- Power (watts or milliwatts as marked)
- dBm
- Power relative to 1 mW in decibels
- dBW
- Power relative to 1 W in decibels
- Z
- Load impedance in ohms (resistive)
- Vrms
- RMS voltage across the load
Worked examples
20 dBm transmitter output
20 dBm = 10^(20/10) = 100 mW, which is 0.1 W and −10 dBW. Into a 50 Ω load, Vrms = √(0.1 × 50) ≈ 2.24 V, or about 3.16 V peak.
5 W handheld radio to dBm
5 W is 5,000 mW, so dBm = 10 × log10(5,000) ≈ 36.99 dBm and dBW = 6.99. Across 50 Ω that is √(5 × 50) ≈ 15.81 V RMS.
Weak received signal of −30 dBm
−30 dBm is 10^(−3) mW = 0.001 mW, or one microwatt, which equals −60 dBW. On a 50 Ω input it produces only about 7.07 mV RMS.
Frequently asked questions
How do I convert dBm to watts?+
Divide the dBm value by 10, raise 10 to that power to get milliwatts, then divide by 1,000. For example 30 dBm → 10³ = 1,000 mW = 1 W, and 40 dBm = 10 W.
What is the difference between dBm and dBW?+
dBm is referenced to 1 milliwatt and dBW to 1 watt. Since 1 W is 30 dB above 1 mW, you convert by subtracting 30: 30 dBm = 0 dBW and 0 dBm = −30 dBW.
What does 0 dBm mean?+
0 dBm is exactly 1 milliwatt. Positive dBm values are more than 1 mW (10 dBm = 10 mW) and negative values are less (−10 dBm = 0.1 mW). It is not zero power.
Why does +3 dB double the power?+
Because 10 × log10(2) ≈ 3.01. Each +3 dB roughly doubles the power and each −3 dB halves it, while +10 dB is exactly ten times the power.
Does impedance change the dBm to watts conversion?+
No. dBm and watts are both power units, so the conversion is the same for any impedance. Impedance only matters when you convert power to voltage (or dBm to dBµV/dBV), which is why the tool asks for it.
What is a good Wi-Fi signal strength in dBm?+
Received Wi-Fi signals are tiny: around −50 dBm (10 nW) is excellent, −67 dBm is a common minimum for reliable voice and video, and below about −80 dBm connections become unreliable.