Watts to dBm Calculator

dBm is an absolute power level referenced to one milliwatt. It is not the same as dB, which usually expresses a ratio between two quantities.

A tenfold power increase adds 10 dB: 1 mW is 0 dBm, 10 mW is 10 dBm, 100 mW is 20 dBm and 1 W is 30 dBm. Doubling power adds about 3.0103 dB.

Negative dBm values are valid and represent positive powers below one milliwatt. For example, −30 dBm is one microwatt. Negative watts remain physically invalid for this conversion.

Use power, not voltage, unless impedance and the correct RMS relationship are known. A voltage reading alone cannot be assigned a unique dBm value across arbitrary loads.

The calculation does not include antenna gain, cable loss or modulation. Keep those link-budget quantities separate and apply their signed dB values at the appropriate stage.

Record enough context to reproduce this watts-to-dBm conversion: input source, unit, date and any method choices. A bare number is easy to misread when it is copied into a report or compared later.

Round for communication only after the calculation. The endpoint formatting improves readability but does not increase the accuracy of the measurements, assumptions or source data.

Logarithmic units make multiplication in linear power appear as addition. If an amplifier ideally multiplies power by 100, the level rises by 20 dB because 10 log10(100) equals 20. This relationship is convenient in link budgets.

When combining independent signal powers, convert each dBm value back to milliwatts, add the linear powers, and convert the total to dBm. Directly adding two dBm readings is generally wrong. Two equal uncorrelated 10 dBm powers total about 13.01 dBm, not 20 dBm.

A receiver sensitivity such as −100 dBm is a power threshold, while an antenna gain stated in dBi is a ratio referenced to an isotropic radiator. Their similar-looking suffixes describe different references and should retain their units throughout a link calculation.

Instrument bandwidth, detector mode and calibration affect a measured power. The converter assumes the watt value is already the correct quantity; it cannot decide whether a peak, average or channel-power reading is appropriate.

Scientific notation avoids lost detail for tiny inputs. One nanowatt is 0.000000001 W and converts to −60 dBm. Enter the positive decimal value supported by the browser and check that it was not rounded to zero before submission.

Rounding in logarithmic units can matter when a value is later converted back to linear power. Keep the unrounded dBm result during a longer link-budget calculation and round only the final display. Four decimal places on this page are useful for arithmetic checks, but most physical measurements will justify fewer digits.

Prefixes must be resolved before taking the logarithm. A kilowatt is 1,000 watts and a microwatt is one-millionth of a watt. Writing the value in watts first gives a clear audit trail and prevents a hidden factor-of-1,000 error.