dBm to Watts Calculator

Use this calculator to convert an absolute power level in dBm to milliwatts or watts, or to convert linear power in watts back to dBm. The conversion is useful for RF, microwave, wireless, optical, and communication-system calculations that span a wide range of power levels.

dBm is referenced to 1 milliwatt. Therefore, 0 dBm equals 1 mW, 10 dBm equals 10 mW, 20 dBm equals 100 mW, and 30 dBm equals 1 W.

How to Use the dBm to Watts Converter

1. Choose whether the known value is in dBm, milliwatts, or watts.

2. Enter the known power value.

3. Run the calculation to display the equivalent power units.

4. Keep a suitable number of significant digits for the source measurement.

Linear power must be greater than zero when converting watts or milliwatts to dBm because the logarithm of zero is undefined. As power approaches zero, the corresponding dBm value approaches negative infinity.

What Is dBm?

dBm expresses a power level on a base-10 logarithmic scale with a reference power of 1 mW. It is commonly used because a short range of dBm values can represent linear powers ranging from fractions of a picowatt to kilowatts.

The reference is part of the meaning: 0 dBm does not mean zero power. It means that the power equals the 1 mW reference.

What Is a Watt?

The watt, symbol W, is the SI unit of power. One watt equals one joule of energy transferred per second:

1 W = 1 J/s

Common submultiples include the milliwatt, microwatt, nanowatt, and picowatt:

• 1 mW = 10⁻³ W

• 1 µW = 10⁻⁶ W

• 1 nW = 10⁻⁹ W

• 1 pW = 10⁻¹² W

dBm to Watts Formula

Convert dBm to milliwatts with:

P_mW = 10^P_dBm/10

Convert dBm directly to watts with:

P_W = 10^{(P_dBm − 30)/10}

where P_dBm is the power level in dBm, P_mW is power in milliwatts, and P_W is power in watts.

Example: Convert 20 dBm to Watts

P_W = 10^{(20 − 30)/10} = 10⁻¹ = 0.1 W

Therefore, 20 dBm equals 100 mW or 0.1 W.

Example: Convert 23 dBm to Watts

P_W = 10^{(23 − 30)/10} = 0.199526 W

Therefore, 23 dBm is approximately 199.526 mW.

Example: Convert −85 dBm to Watts

P_W = 10^{(−85 − 30)/10} = 3.16228 × 10⁻¹² W

Therefore, −85 dBm is approximately 3.16228 pW.

Watts to dBm Formula

Convert watts to dBm with:

P_dBm = 10 log₁₀(P_W/0.001 W)

The equivalent convenient form is:

P_dBm = 10 log₁₀(P_W) + 30

When power is entered in milliwatts:

P_dBm = 10 log₁₀(P_mW)

Example: Convert 2 W to dBm

P_dBm = 10 log₁₀(2) + 30 = 33.0103 dBm

Example: Convert 50 µW to dBm

First convert 50 µW to 0.05 mW:

P_dBm = 10 log₁₀(0.05) = −13.0103 dBm

dBm to Watts Conversion Table

dBm, dB, and dBW

Because 1 W is 1000 times 1 mW, the relationship is:

P_dBm = P_dBW + 30

Thus 30 dBm equals 0 dBW, and both represent 1 W.

How a dB Change Affects Power

A power-level difference in dB corresponds to a linear power ratio:

Power ratio = 10^ΔdB/10

A 3 dB increase is only an approximation to doubling. An exact factor of two corresponds to approximately 3.0103 dB.

Adding Gains and Power Levels

Add Gain or Loss in dB to a dBm Level

Gains and losses expressed as dB ratios can be added algebraically to an absolute dBm power level. For example:

10 dBm + 20 dB gain − 3 dB loss = 27 dBm

The resulting 27 dBm is approximately 0.501187 W.

Do Not Directly Add Two dBm Values

Two independent power levels must normally be converted to watts or milliwatts, added in linear units, and then converted back to dBm. For two 30 dBm powers:

30 dBm = 1 W; 1 W + 1 W = 2 W; 2 W = 33.0103 dBm

The result is not 60 dBm. When combining coherent signals, phase and the combining network can also affect the measured result, so simple power addition may not describe the complete circuit behavior.

dBm to Voltage Requires an Impedance

dBm describes power, not voltage. To calculate voltage, the load impedance and voltage definition must be known. For power delivered to a purely resistive load:

V_RMS = √(P_W × R)

For a sine wave only:

V_peak = √2 × V_RMS
   V_pp = 2√2 × V_RMS

In a 50 Ω resistive system, 0 dBm corresponds to approximately 0.223607 V RMS and 0.632456 V peak-to-peak for a sine wave. A different impedance produces a different voltage for the same power.

Common Mistakes

• Treating 0 dBm as zero watts instead of 1 mW.

• Using 20 rather than 10 as the multiplier for a power ratio in decibels.

• Forgetting the 30 dB offset between dBm and dBW.

• Reading 20 dBm as 100 W instead of 100 mW.

• Adding two dBm power levels directly.

• Converting dBm to voltage without specifying impedance.

• Using peak or peak-to-peak voltage where RMS voltage is required.

• Reporting excessive digits when the measured power has limited accuracy.

dBm Conversion FAQ

How many watts is 0 dBm?

0 dBm equals 1 mW, which is 0.001 W.

How many watts is 10 dBm?

10 dBm equals 10 mW or 0.01 W.

How many watts is 20 dBm?

20 dBm equals 100 mW or 0.1 W.

How many dBm is 1 watt?

1 W equals 1000 mW, so 10 log₁₀(1000) = 30 dBm.

Can dBm be negative?

Yes. A negative dBm value represents a positive power below 1 mW. For example, −30 dBm equals 1 µW. It does not represent negative physical power.

Is dBm a frequency-dependent conversion?

The mathematical conversion between dBm and watts does not depend on frequency. However, the actual delivered or measured power can depend on frequency response, impedance matching, cable loss, calibration, and instrument bandwidth.

Why is 3 dB called a doubling of power?

A 3 dB increase multiplies power by approximately 1.99526, which is close to two. The exact dB value for a factor of two is 10 log₁₀(2), approximately 3.0103 dB.

References

• NIST Technical Note 1850, Appendix B: Linear and Logarithmic Power Levels

• NIST Glossary: Decibels Referenced to One Milliwatt

• NIST Guide to the SI: Decibel and Logarithmic Power Ratios

• Rohde and Schwarz Application Note: dB Calculator