dBm, watts and volts converter

dBm is a power, not a voltage: it is how many decibels a signal sits above one milliwatt. Watts follow from it with nothing else needed. Volts do not — a power turns into a voltage only once you say what impedance it is developed across, and the answer moves by 10.8 dB between the 50 Ω of a radio bench and the 600 Ω of an audio one. That impedance is a field here rather than a hidden constant.

How it is calculated

dBm = 10 log₁₀(P ÷ 1 mW) · P = V²rms ÷ Z · Vpeak = √2 × Vrms · Vpp = 2√2 × Vrms

The first line is the definition and needs no impedance. The second introduces one, and everything below it inherits that choice. The last two hold for a sine wave and nothing else — a square wave has the same rms as its peak, and a modulated carrier has a peak-to-peak far above what its average power suggests.

Source: Texas Instruments — Analog Engineer's Pocket Reference, fifth edition (SLYW038D, April 2025): Power measured (dBm) = 10log(Power measured (W) / 1 mW), equation (65), and RMS for full wave rectified sine wave, equation (44): VRMS = VPEAK / √2

Questions people ask

Why does dBm need an impedance to become volts?

Because it does not contain one. dBm is defined against a milliwatt of power, and a milliwatt is 0.224 V across 50 Ω but 0.775 V across 600 Ω. Any converter that turns dBm into volts without asking has quietly picked one for you, and it is nearly always 50 Ω. If your signal is on an audio line, that answer is wrong by a factor of 3.46 in voltage.

Which impedance is mine?

50 Ω for radio, test gear and most coaxial signal paths. 75 Ω for video, aerial downleads and SDI. 600 Ω for telephony and older audio line levels, which is where the 0.775 V of dBu comes from. Modern audio gear is mostly bridging — a high-impedance input across a low-impedance output — so no real power flows and dBu, referred to 0.775 V alone, is the honest unit rather than dBm.

Is peak to peak really 2.828 times the rms?

For an undistorted sine wave, yes: the peak is √2 times the rms and peak to peak is twice the peak. For anything else the ratio changes. A square wave has a peak equal to its rms, noise has no fixed ratio at all, and a modulated carrier can reach several times what its average power implies. The peak figures on this page assume a sine.

What is the difference between dBm and dBu?

dBm is referred to one milliwatt of power; dBu is referred to 0.775 V regardless of impedance. They coincide only at 600 Ω, because 0.775 V across 600 Ω is exactly one milliwatt — which is why the two were used interchangeably back when everything really was 600 Ω, and why they stopped agreeing once it was not.

How much is 3 dB, really?

Twice the power and 1.41 times the voltage. 10 dB is ten times the power and 3.16 times the voltage. The reason the same number of decibels means two different multipliers is that power goes as voltage squared, so power ratios take 10 log and voltage ratios take 20 log.

Sources

The documents this page reads its numbers out of, linked so you can check them yourself.

1. Texas Instruments — Analog Engineer's Pocket Reference, fifth edition (SLYW038D, April 2025): Power measured (dBm) = 10log(Power measured (W) / 1 mW), equation (65), and RMS for full wave rectified sine wave, equation (44): VRMS = VPEAK / √2
2. Mini-Circuits — dBm - volts - watts conversion (50-ohm system), chart dg03-110: 0 dBm is .225 V and 1.0 mW
3. ITU-T Recommendation G.100.1 (06/2015), The use of the decibel and of relative levels in speechband telecommunications, clause 8.3: The use of "dBm" in this application is only correct if the test impedance is 600 Ω resistive since 0.775 V across 600 Ω results in the reference active power of 1 mW

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