dBm to Watts Converter
Convert between dBm, watts and voltage at any impedance, with dBW, dBµV and dBmV alongside.
Step-by-step
- Enter valid values to begin.
Assumes a sinusoidal (CW) signal into a purely resistive, matched load Z, so P = Vrms² / Z. Voltage, current and dBµV results change with impedance, but dBm and watts do not. Peak and peak-to-peak values use the sinusoidal factors √2 and 2√2; modulated or noise-like signals have a different crest factor. The optional sum adds power (not dB values) and assumes the two signals are uncorrelated, for example at different frequencies.
dBm to Watts: Why Radio People Count in Decibels
Core Engineering Principles
A decibel is a ratio, so dBm means decibels relative to one milliwatt. Zero dBm is 1 mW, every +10 dB multiplies power by ten, and every +3 dB roughly doubles it. We use logs because RF signals span absurd ranges: a 100 W transmitter at +50 dBm and a receiver input at −100 dBm differ by a factor of 10¹⁵. The conversion is P = 1 mW × 10^(dBm/10).
Power does not depend on impedance, but voltage does, and that is where mistakes start. For a sine wave, P = V²/Z, so 20 dBm into 50 Ω is 2.236 V rms, while the same power into 75 Ω is 2.739 V rms. That is also why dBµV has an impedance-dependent offset: dBµV = dBm + 90 + 10 log₁₀(Z), which is 106.99 at 50 Ω and 108.75 at 75 Ω. Quote “107” for 50 Ω and you will be wrong at 75 Ω by almost 2 dB. Never add decibel values for two signals. Convert to watts, add, and convert back: two 20 dBm tones make 23.01 dBm, but 20 dBm plus 10 dBm is only 20.41 dBm.
Vrms = √(PZ) • Vpp = 2√2 Vrms • dBµV = dBm + 90 + 10 log10(Z)
NEC & Standard References
IEEE Std 100 and IEC 60050 (IEV) define the decibel and the logarithmic ratio quantities. ITU-R Recommendation V.574 covers the use of the decibel and neper in telecommunications. IEC 61169 defines RF coaxial connector families and MIL-STD-348 their interface dimensions, in the 50 Ω and 75 Ω practice that most test gear follows. Regulators such as the FCC (47 CFR Parts 15 and 97) and ETSI (EN 300 series) state transmitter limits as power or field strength, often in dBm or EIRP; verify current limits for your band.1. P = 1 mW × 102 = 100 mW = 0.1 W, which is −10 dBW.
2. Vrms = √(0.1 × 50) = 2.236 V, Vpeak = 3.162 V and Vpp = 6.325 V.
3. Irms = 2.236 / 50 = 44.7 mA.
4. dBµV = 20 + 90 + 16.99 = 126.99 dBµV, and dBmV = 66.99.
5. Many analyzers are damaged by input levels near +30 dBm, and some specify far less. With a 30 dB attenuator the input sees −10 dBm, which is 100 µW, a comfortable level.
- Count the attenuator’s power rating, not just its dB. A 30 dB pad on a 100 W transmitter must absorb nearly 100 W as heat.
- Keep your impedance straight. A 50 Ω meter reading on a 75 Ω system is off by 1.76 dB in dBµV.
- Peak power is not average power. Modulated signals swing above their average and can overload an analyzer front end.
- Never touch a live RF connector or antenna. A few watts at VHF can burn skin, and high power is an exposure hazard.
- Don’t trust a meter near its floor. Close to −100 dBm, noise and bandwidth settings dominate the reading.