dBm to Watts and Milliwatts: Power Conversions

This calculator converts power expressed in dBm, watts, or milliwatts into the other two units. Enter one power value and select its unit; the results show the equivalent power in dBm, W, and mW.

Formula

The conversions use logarithms because dBm is a logarithmic measure of power, while watts and milliwatts are linear measures. The reference for dBm is one milliwatt:

• Power in dBm: P_dBm = 10 log₁₀(P_mW)
• Power in milliwatts: P_mW = 10^P_dBm/10
• Power in watts: P_W = P_mW / 1000
• From watts to dBm: P_dBm = 10 log₁₀(1000 P_W)
• From dBm to watts: P_W = 10^{(P_dBm−30)/10}

Here, P_dBm is power in dBm, P_mW is power in milliwatts, and P_W is power in watts. The logarithm is base 10. The calculator’s input value is interpreted according to the selected unit; the formulas above describe each conversion independently.

Worked Example

The defaults are 0 dBm. Substitute that value into the conversion formulas:

1. Convert to milliwatts: 10^0/10 = 1 mW.
2. Convert to watts: 1 / 1000 = 0.001 W.
3. Check the dBm result from milliwatts: 10 log₁₀(1) = 0 dBm.

The calculator therefore returns 0.00 dBm, 0.001000 W, and 1.000 mW. The displayed digits are rounded; the underlying conversion is exact for these values.

What dBm, Watts, and Milliwatts Mean

A watt is the SI unit of power. One watt equals 1,000 milliwatts, so watts and milliwatts use the same linear scale: doubling the power doubles the numeric value. Milliwatts are convenient for small signal levels because many radio and communications powers are much less than one watt.

dBm is an absolute power level measured relative to 1 mW. It is not the same as dB, which expresses a ratio or difference between levels and has no fixed power reference by itself. A reading of 0 dBm means 1 mW, not zero power. Negative dBm values represent powers below 1 mW; positive values represent powers above 1 mW.

Useful Conversion Benchmarks

Each increase of 10 dB corresponds to ten times the power. An increase of about 3 dB is approximately a doubling, while a decrease of about 3 dB is approximately halving. These are useful mental checks, not substitutes for precise calculations.

• −30 dBm = 0.001 mW = 1 µW.
• −10 dBm = 0.1 mW = 0.0001 W.
• 0 dBm = 1 mW = 0.001 W.
• 10 dBm = 10 mW = 0.01 W.
• 20 dBm = 100 mW = 0.1 W.
• 30 dBm = 1,000 mW = 1 W.

For example, 23 dBm is 10^2.3, or about 199.5 mW, which is about 0.1995 W. The conversion is exponential: adding 10 dB multiplies power by ten; it does not add 10 mW.

How to Use the Converter

Type a power value and choose whether it is in dBm, W, or mW. The tool converts the selected value to all three units. For instance, if a specification gives 0.5 W, select W and enter 0.5; the equivalent is 500 mW and about 27.0 dBm.

Use the unit stated by the source, and check whether a value is a power level or a power difference. dBm is appropriate for an absolute power referenced to 1 mW. A gain or loss stated in dB should not be entered as though it were a dBm power level.

Common Mistakes and Limits

• Confusing dB with dBm: dB describes a ratio or change; dBm gives an absolute level with a 1 mW reference.
• Taking a logarithm of zero: zero watts cannot be represented as a finite dBm value. The logarithm is undefined at zero, and the dBm level approaches negative infinity as positive power approaches zero.
• Mixing watts and milliwatts: 1 W is 1,000 mW. A factor-of-1,000 error changes the dBm result by 30 dB.
• Rounding too early: retain precision through intermediate steps, then round the final display to the precision needed.
• Assuming power alone describes a signal: this conversion does not account for antenna gain, cable loss, impedance, frequency, or measurement conditions.

The conversion applies to power values expressed in the selected units. It does not by itself convert voltage to power: that requires additional information, such as the load resistance and whether the voltage is RMS. Nor does it determine whether a particular power is suitable for equipment or a given application.

Frequently Asked Questions

How many milliwatts are in 1 dBm?
1 dBm is 10^1/10 mW, approximately 1.259 mW. It is slightly more than 1 mW because 0 dBm, not 1 dBm, equals exactly 1 mW.

What is 0 dBm in watts?
0 dBm equals 1 mW, which is 0.001 W.

How do I convert watts to dBm?
Multiply the power in watts by 1,000 to get milliwatts, then calculate 10 log₁₀(power in milliwatts). For example, 1 W equals 1,000 mW and therefore 30 dBm.

Can a dBm value be negative?
Yes. A negative dBm value means the power is less than 1 mW. For example, −10 dBm is 0.1 mW.

Why can’t zero watts be expressed as a finite dBm number?
The dBm formula takes the logarithm of power. The logarithm of zero is undefined, so zero power has no finite dBm value.

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in value | Power value | - | 0 | 0
sel unit | Input unit | dBm=1, W=2, mW=3
out dbm | Power | dBm | (unit=1)*value+(unit=2)*(10*log10(value*1000))+(unit=3)*(10*log10(value)) | 2
out watts | Power | W | (unit=1)*(10^((value-30)/10)/1000)+(unit=2)*value+(unit=3)*value/1000 | 6
out milliwatts | Power | mW | (unit=1)*10^(value/10)+(unit=2)*value*1000+(unit=3)*value | 3