dBm to Watts Calculator

dbm

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Power in watts (W)

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Table of Contents

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What is dBm to Watts?

This calculator converts dBm, a logarithmic power level referenced to one milliwatt, into watts. The “m” in dBm identifies the 1 mW reference. A value of 0 dBm equals 1 milliwatt or 0.001 watt; 30 dBm equals 1 watt. Each increase of 10 dB multiplies power by ten, while each decrease of 10 dB divides it by ten.

dBm describes absolute power, unlike dB by itself, which normally expresses a ratio. The conversion applies to power. Voltage conversion needs impedance and uses a different relationship, so a dBm reading cannot be turned into volts without more information.

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How to Use This Calculator

  1. Enter the power level in dBm. Positive, zero and negative values are meaningful.
  2. Select Calculate.
  3. Read the output in watts.
  4. For very small signals, use scientific notation or smaller power units when helpful.

The page divides dBm by 10, raises 10 to that power, and divides by 1,000 to change milliwatts to watts. It retains up to twelve decimal places, so −50 dBm displays as 0.00000001 W. Extremely small values may still be clearer in scientific notation.

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Formula Used

Power in milliwatts = 10dBm ÷ 10

Power in watts = 10dBm ÷ 10 ÷ 1000

The inverse is dBm = 10 log10(1000 × watts), for positive power. These equations use a base-10 logarithm and a 1 mW reference.

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Variables Explained

TermMeaning
dBmAbsolute logarithmic power level relative to 1 milliwatt.
dBA dimensionless logarithmic ratio; it lacks the fixed milliwatt reference by itself.
mWMilliwatt, equal to one-thousandth of a watt.
WWatt, the SI derived unit of power.
10xBase-10 exponentiation that reverses the logarithmic dBm scale.
Reference power1 mW, the level defined as 0 dBm.
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Worked Example

Convert 20 dBm to watts:

  1. Divide by 10: 20 ÷ 10 = 2.
  2. Raise 10 to that power: 10² = 100 mW.
  3. Divide by 1,000: 100 ÷ 1,000 = 0.1 W.

Useful checkpoints follow the same decade pattern: −30 dBm = 0.001 mW = 0.000001 W; 0 dBm = 0.001 W; 10 dBm = 0.01 W; 30 dBm = 1 W; and 40 dBm = 10 W. A 3 dB change is approximately a factor of two, but 10 dB is exactly a factor of ten under the formula.

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Why Use This Calculator?

The result is power expressed on a linear watt scale. Use it to compare equipment limits, transmitter output or received-signal power only when the original dBm value refers to the same measurement point and bandwidth. Cable loss, antenna gain and amplifier gain are usually combined as dB gains or losses before converting the final dBm value.

A negative dBm value does not mean negative power. It means positive power below the 1 mW reference. Likewise, adding dBm values is generally invalid; add gains and losses in dB to a dBm level. Because this page rounds to four decimal places, independently calculate or retain more precision for levels below about −40 dBm.

When combining independent powers, first convert each dBm value to a linear unit, add the watt or milliwatt values, and convert the total back if needed. For example, two equal 0 dBm sources contain 1 mW each, so their ideal summed power is 2 mW, about 3.01 dBm rather than 0 dBm or 0 dBm squared. Whether powers can physically be combined also depends on phase, circuit topology and measurement setup.

Receiver sensitivity, conducted transmitter power and radiated power are not interchangeable labels. Antenna gain and feed-line loss affect EIRP or ERP, and regulations may specify which quantity and bandwidth to use. Preserve the measurement context along with the converted number.

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Common Mistakes

    • Using 20 instead of 10 in the power formula; 20 applies to certain amplitude ratios.
    • Forgetting to divide milliwatts by 1,000 to obtain watts.
    • Reading negative dBm as negative watts.
    • Confusing dBm with dBW; 0 dBW equals 1 W or 30 dBm.
    • Adding two absolute dBm powers directly.
    • Assuming a rounded display of 0 means exactly zero power.
    • Converting to voltage without specifying load impedance.
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Frequently Asked Questions

What is 0 dBm in watts?

Zero dBm is 1 milliwatt, which equals 0.001 watt.

Why can dBm be negative?

The logarithm is negative when power is below the 1 mW reference. The corresponding watt value remains positive.

Is 30 dBm equal to 30 watts?

No. Thirty dBm equals 1 watt. Forty dBm equals 10 watts, and 50 dBm equals 100 watts.

How are small signals displayed?

The page retains up to twelve decimal places, so −50 dBm appears as 0.00000001 W. Scientific notation may be clearer for much smaller values.

Can I convert dBm directly to volts?

Not without impedance. Voltage depends on both power and load resistance, so the impedance and signal convention must be known.

What is the difference between dBm and dBW?

dBm uses 1 mW as reference; dBW uses 1 W. For the same power, dBm is 30 greater than dBW. State the reference explicitly when copying specifications. A transmitter listed at 10 dBW is 40 dBm and 10 W; treating the numerical dBW value as dBm would create a thousandfold power error.

How do I combine two power levels stated in dBm?

Convert each level to milliwatts or watts, add the linear powers when the physical setup permits that addition, and convert the total back to dBm if needed. Directly adding the two dBm numbers does not produce total power.

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Sources & References

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