dB to mW: Convert dBm to Milliwatts and dBW to Watts

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A direct dB to mW or dB to W conversion is not possible from a plain dB value alone. Plain dB expresses a power ratio, while dBm and dBW express power relative to stated reference levels. For a usable power result, the reference must be known.

dBm uses 1 mW as its reference, and dBW uses 1 W. The formulas below convert each logarithmic level to linear power and back, including negative, zero, and positive values.

dB to mW: Why dB, dBm, and dBW are different

Plain dB describes the ratio between two power levels:

Power ratio = 10^(dB/10)

For example, a 10 dB increase means the output power is 10 times the input power, while a -10 dB change means the output power is one-tenth of the input power. Because dB is a ratio, it does not identify an absolute number of milliwatts or watts without a reference power.

With a known reference, the absolute output power can be calculated as:

Output power = Reference power × 10^(dB/10)

For example, a 6 dB gain applied to a 2 mW input produces approximately 7.96 mW:

2 mW × 10^(6/10) = 2 × 3.981 = 7.96 mW

dBm is an absolute power level referenced to 1 mW:

  • 0 dBm = 1 mW
  • 10 dBm = 10 mW
  • -10 dBm = 0.1 mW

dBW is an absolute power level referenced to 1 W:

  • 0 dBW = 1 W
  • 10 dBW = 10 W
  • -10 dBW = 0.1 W

Therefore, a plain dB value should not be labeled as a power level. A statement such as “-3 dB equals 0.5 W” is incomplete unless the reference power is supplied. A statement such as “-3 dBm equals approximately 0.501 mW” is complete because dBm already defines its reference.

Convert dBm to milliwatts with the right formula

Forward conversion: dBm to mW

To convert dBm to milliwatts, use:

Power (mW) = 10^(power (dBm)/10)

The result is in milliwatts because the dBm reference is 1 mW. The exponent is the dBm value divided by 10.

  • Negative value: -20 dBm = 10^(-20/10) = 10^-2 = 0.01 mW, or 10 µW.
  • Zero value: 0 dBm = 10^(0/10) = 1 mW.
  • Positive value: 10 dBm = 10^(10/10) = 10 mW.
  • Higher positive value: 20 dBm = 10^(20/10) = 100 mW.

Another useful example is -3 dBm:

10^(-3/10) = 10^-0.3 ≈ 0.501 mW

For levels expressed in dBm but needed in watts, first calculate milliwatts and divide by 1,000. A direct formula is also available:

Power (W) = 10^((power (dBm) – 30)/10)

For example, 30 dBm equals:

10^((30 – 30)/10) = 1 W

Reverse conversion: mW to dBm

To convert milliwatts back to dBm, use the base-10 logarithm:

Power (dBm) = 10 × log10(power (mW))

  • Negative result: 0.01 mW gives 10 × log10(0.01) = -20 dBm.
  • Zero result: 1 mW gives 10 × log10(1) = 0 dBm.
  • Positive result: 10 mW gives 10 × log10(10) = 10 dBm.

For a non-power-of-ten value, 2 mW converts as follows:

10 × log10(2) ≈ 3.01 dBm

Similarly, 0.5 mW is approximately -3.01 dBm. The logarithm automatically produces a negative dBm value for any power below 1 mW and a positive value for any power above 1 mW.

dB to W: Convert dBW to watts

Forward conversion: dBW to W

To convert dBW to watts, use:

Power (W) = 10^(power (dBW)/10)

Because dBW is referenced to 1 W, a dBW value of zero represents exactly 1 W.

  • Negative value: -20 dBW = 10^(-20/10) = 0.01 W, or 10 mW.
  • Zero value: 0 dBW = 10^(0/10) = 1 W.
  • Positive value: 10 dBW = 10^(10/10) = 10 W.
  • Higher positive value: 20 dBW = 10^(20/10) = 100 W.

For a value that is not a multiple of 10, -3 dBW converts to:

10^(-3/10) = 10^-0.3 ≈ 0.501 W

The direct conversion from dBW to milliwatts is:

Power (mW) = 1,000 × 10^(power (dBW)/10)

For example, 0 dBW equals 1 W, which is 1,000 mW. Likewise, -30 dBW equals 0.001 W, which is 1 mW.

Reverse conversion: W to dBW

To convert watts back to dBW, use:

Power (dBW) = 10 × log10(power (W))

  • Negative result: 0.01 W gives 10 × log10(0.01) = -20 dBW.
  • Zero result: 1 W gives 10 × log10(1) = 0 dBW.
  • Positive result: 10 W gives 10 × log10(10) = 10 dBW.

For 0.5 W:

10 × log10(0.5) ≈ -3.01 dBW

For 100 W:

10 × log10(100) = 20 dBW

As with dBm, a wattage below the reference level produces a negative logarithmic value, while a wattage above the reference produces a positive value.

Worked examples: decibels to watts, reverse conversions, and the 30 dB offset

Negative, zero, and positive examples

The following examples show how the two reference systems behave at common values:

  • -30 dBm: 10^(-30/10) = 0.001 mW = 1 µW.
  • -10 dBm: 10^(-10/10) = 0.1 mW.
  • 0 dBm: 10^(0/10) = 1 mW.
  • 10 dBm: 10^(10/10) = 10 mW.
  • 30 dBm: 10^(30/10) = 1,000 mW = 1 W.
  • -30 dBW: 10^(-30/10) = 0.001 W.
  • -10 dBW: 10^(-10/10) = 0.1 W.
  • 0 dBW: 10^(0/10) = 1 W.
  • 10 dBW: 10^(10/10) = 10 W.
  • 30 dBW: 10^(30/10) = 1,000 W.

For reverse conversions, 0.001 mW is -30 dBm, 1 mW is 0 dBm, and 1,000 mW is 30 dBm. In watts, 0.001 W is -30 dBW, 1 W is 0 dBW, and 1,000 W is 30 dBW.

How the 30 dB dBm-to-dBW offset works

dBm and dBW use different reference powers: 1 mW and 1 W. Since 1 W equals 1,000 mW, and:

10 × log10(1,000) = 30 dB

the two scales have a fixed 30 dB offset:

Power (dBm) = Power (dBW) + 30

Power (dBW) = Power (dBm) – 30

Examples include:

  • 0 dBW = 30 dBm: both represent 1 W.
  • -10 dBW = 20 dBm: both represent 0.1 W, or 100 mW.
  • -30 dBW = 0 dBm: both represent 1 mW.
  • 10 dBW = 40 dBm: both represent 10 W.

The offset changes only the numerical label, not the physical power. A conversion involving plain dB still requires a reference. For example, a gain of 30 dB applied to 1 mW produces 1 W, but the same 30 dB gain applied to 10 mW produces 10 W.