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dBm to watts converter

This dBm to watts converter works in both directions. Example: 20 dBm equals 100 mW, which is 0.1 W; across 50 Ω that is 2.23607 V.

Example: What is given: Level in dBm → watts · Power level 20 dBm · Resistance Z 50 Ω → Result: 20 dBm = 0.1 W. Source: IEC 80000-3 / ITU-R V.574: Use of the decibel and the neper (Leistungspegel L = 10·lg(P/P₀)). Updated: .

dBm
Relative to 1 mW. Negative values are possible (0 dBm = 1 mW).
Ω
Used for the voltage; usually 50 Ω in RF, 75 Ω in video.
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Result

Result
20 dBm = 0.1 W
All representations
QuantityValue
Level in dBm20 dBm
Level in dBW-10 dBW
Power in W0.1 W
Power in mW100 mW
Power in µW100,000 µW
RMS voltage across the resistance2.23607 V (50 Ω)
Calculation steps
  • P = 1 mW · 10^(dBm / 10) = 1 mW · 10^(20 / 10) = 100 mW = 0.1 W.
  • dBW = dBm − 30 = -10.
  • U = √(P·Z) = √(0.1 W · 50 Ω) = 2.23607 V.
dBm is a power level relative to 1 mW (10·log). 3 dB is roughly a doubling of power, 10 dB a factor of 10. The voltage assumes a purely resistive load.

How it is calculated

dBm is a logarithmic power level: it states by how many decibels a power lies above or below 1 milliwatt. It is used in radio, antenna and cable engineering because gains and losses simply add.

Formulas

P = 1 mW · 10^(dBm / 10) and conversely dBm = 10 · log₁₀(P / 1 mW). The level in dBW refers to 1 W: dBW = dBm − 30. The RMS voltage across a resistance Z is U = √(P·Z).

Worked example

20 dBm: P = 1 mW · 10^(20/10) = 100 mW = 0.1 W, which is -10 dBW. Across 50 Ω this gives U = √(0.1 · 50) = 2.23607 V. More fixed points: 0 dBm = 1 mW, 3 dBm ≈ 1.99526 mW (almost doubled), 30 dBm = 1 W and −30 dBm = 1 µW. Conversely, 1 W (1000 mW) is exactly 30 dBm. A receiver at its sensitivity limit of −90 dBm sees only 1 pW.

Frequently asked questions

How do I convert dBm to watts?

Use P = 1 mW · 10^(dBm/10), then divide by 1000 for watts. 20 dBm is 100 mW or 0.1 W.

How many watts is 30 dBm?

30 dBm is exactly 1 W, because 30 dBm = 0 dBW.

Why work in dBm instead of watts?

Because radio signals span many powers of ten, from watts to picowatts. The logarithm turns them into handy numbers, and gains can simply be added.

Sources and legal basis

As of: · Editorial policy

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