dBm ⇄ Watts ⇄ dBW Converter

Understanding the dBm to watts converter — concept and principle

Radio, audio and fibre-optic engineers express power in decibels relative to one milliwatt (dBm), because signal levels span many orders of magnitude, from kilowatt transmitters to receivers detecting a picowatt. This converter turns a dBm value into milliwatts, watts and dBW, and also shows the RMS voltage that power produces across a 50 Ω load.

Each 10 dB step multiplies power by ten; 20 dBm is 100 mW, or −10 dBW.

How it is calculated

Milliwatts: P(mW) = 10^(dBm / 10).
Watts: P(W) = 10^((dBm − 30) / 10), since 1 W = 1000 mW = 30 dBm.
dBW: dBW = dBm − 30 (decibels relative to 1 W).
Voltage into 50 Ω: V = √(P × R) with P in watts and R = 50 Ω.

The reverse conversion is dBm = 10 log₁₀(P in mW).

The principle

A decibel is ten times the base-10 logarithm of a power ratio. Writing powers as logarithms turns multiplication into addition: gains and losses through amplifiers, cables and antennas can simply be added and subtracted. The "m" in dBm fixes the reference at 1 mW, so the number becomes an absolute power rather than a ratio. Key landmarks follow from the definition: +10 dB is ×10, +3 dB is very nearly ×2 (10 log₁₀ 2 ≈ 3.01), and −3 dB is about half. Voltage rises with the square root of power, so ten times the power gives about 3.16 times the voltage.

Worked example

Milliwatts: 10^(20 / 10) = 10² = 100 mW.
Watts: 10^((20 − 30) / 10) = 10⁻¹ = 0.1 W.
dBW: 20 − 30 = −10 dBW.
Voltage: √(0.1 × 50) = √5 ≈ 2.236 V rms.

Each point is the calculator re-run with only “Power (dBm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Power (dBm)	Watts
5	0.0031622777
10	0.01
15	0.0316227766
20	0.1
25	0.316227766
30	1
40	10

How to read the result

0 dBm is 1 mW; 30 dBm is 1 W; 40 dBm is 10 W; 60 dBm is 1 kW. Negative values are fractions of a milliwatt: −30 dBm is 1 µW and −60 dBm is 1 nW. A Wi-Fi access point often transmits around 20 dBm, and received signals in wireless links are commonly in the −30 to −90 dBm range. Into 50 Ω, 0 dBm corresponds to about 0.224 V rms.

Common mistakes

Confusing dBm (absolute power) with dB (a ratio) or dBi (antenna gain).
Using 20 log for power; that rule applies only to voltage or current ratios.
Adding dBm values of two signals directly: two 20 dBm signals combine to about 23 dBm, not 40 dBm.
Applying the 50 Ω voltage to other systems, such as 75 Ω video or 600 Ω audio lines.

Keep in mind: The voltage figure assumes a purely resistive 50 Ω load and RMS values for a steady signal. Peak voltages of modulated signals can be much higher than the RMS figure.

Frequently asked questions

How many watts is 30 dBm?

Exactly 1 W. Every 10 dB step multiplies or divides by ten.

What is the difference between dBm and dBW?

Only the reference: dBW is relative to 1 W, so it is always 30 dB lower than the dBm figure.

Why is dBm used in fibre optics too?

Optical power budgets also chain losses from connectors and fibre length, and adding decibels is simpler than multiplying ratios.

Sources: IEC 60027-3, Letter symbols to be used in electrical technology — Logarithmic and related quantities, and their units · The ARRL Handbook for Radio Communications, chapter on electrical fundamentals

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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