The dBm to watts converter above treats dBm as what it actually is: a power expressed on a base-ten logarithmic scale against a fixed reference of one milliwatt. That reference is the entire point, and it is what separates dBm from a bare decibel figure. A number quoted in dB is a ratio between two powers and means nothing on its own. A number quoted in dBm is an absolute power, because the denominator of the ratio has already been fixed at one milliwatt by definition.
Arb Digital publishes free calculators that make the reference explicit rather than burying it, because the reference is where nearly every dBm mistake starts. This page converts in both directions, applies an optional system gain or loss so you can see why radio engineers work in dB at all, and reports the RMS voltage the resulting power represents across an impedance you choose. It is deliberately narrower than a general unit tool: the power converter rescales watts into horsepower and BTU per hour by linear factors, while this page crosses between a logarithmic scale and a linear one.
What This dBm to Watts Converter Does
You give it one power value and say which unit it is in. It returns that same power in dBm, dBW, watts and milliwatts simultaneously, so you never have to run a second conversion to check the first. If you enter a gain or loss in dB, that figure is added on the logarithmic scale before the result is reported, which is the correct order of operations and the reason a chain of amplifiers and cables can be totalled with addition instead of multiplication.
The fourth grid figure is the RMS voltage that power would produce across the impedance in the last field. That conversion is not part of the dBm definition and it depends entirely on the impedance, which is why it sits in its own field with a default rather than being assumed silently. Fifty ohms is the near-universal convention for RF test equipment and radio systems; seventy-five ohms is the convention for video and cable distribution. Enter something else and the voltage figure follows, while the three power figures do not move at all.
The hero figure auto-scales its prefix. Receiver sensitivities live down around minus a hundred dBm, which is a hundred femtowatts, and broadcast transmitters live up in the tens of kilowatts. Printing both in bare watts would give you a screen full of zeroes, so the tool picks a sensible SI prefix and tells you which one it used.
How to Use It
- Enter your value and pick its unit. The unit dropdown is what tells the tool whether your number is on the logarithmic scale or the linear one. Getting this wrong is the single most common error, because 20 dBm and 20 W differ by a factor of two hundred.
- Add gain or loss only if you want it applied. Leave the field at zero to convert your input exactly as entered. Enter minus three to see the power after a three-decibel cable loss, or plus twenty to see it after a twenty-decibel amplifier.
- Set the impedance if you need the voltage. The default of fifty ohms suits most RF work. If you are not working into a matched load, the voltage figure will not describe your circuit and you should ignore it.
- Read all four supporting figures. They are the same power four ways. If one of them looks wrong, you have almost certainly entered the input in the wrong unit.
- Use the presets as sanity checks. A typical Wi-Fi transmitter and a weak receiver input bracket most of the range you will meet in practice, and they give you a feel for how enormous the span between them really is.
The Formula: How dBm Converts to Watts
The definition runs in one direction and inverts cleanly. To go from power to dBm: PdBm = 10 × log10(P ÷ 1 mW), with P expressed in milliwatts. To go back: P in milliwatts = 10(PdBm ÷ 10). Watts are then milliwatts divided by a thousand. The factor of ten in front, rather than twenty, is what marks this as a power ratio; that distinction is set out formally in NIST Special Publication 811, the guide for the use of the International System of Units, which covers how logarithmic quantities are written and why the reference must always be stated.
Work the default through by hand. The input is 20 dBm. Divide by ten to get two, raise ten to that power to get one hundred, and the units are milliwatts, so the power is 100 mW or 0.1 W. In dBW that is 20 minus 30, which is minus 10 dBW — a negative number, correctly, because a tenth of a watt is less than the one-watt reference. The RMS voltage across fifty ohms is the square root of the power times the impedance: √(0.1 × 50) = √5 = 2.236 V. Every one of those figures appears in the panel above.
The relationship between dBm and dBW is a fixed offset of exactly thirty, in every case, with no exceptions. That falls straight out of the definition: changing the reference from one milliwatt to one watt divides the ratio by a thousand, and ten times the logarithm of a thousand is thirty. If you ever see a conversion where the offset is not thirty, something upstream is wrong. The underlying units — the watt and the SI prefixes milli and kilo that this page moves between — are defined in the BIPM SI Brochure, the international reference for the SI, currently in its ninth edition.
Why Radio Engineering Uses a Logarithmic Scale at All
The honest answer is that the numbers are otherwise unusable. A mobile handset might transmit at a quarter of a watt and successfully receive a signal of one ten-billionth of a microwatt. Written linearly that is a range spanning sixteen orders of magnitude, and no engineer wants to carry that many zeroes through a design review. On the dBm scale the same span runs from about +24 to about −110, which fits on a single axis and in a single head.
The second reason is arithmetic. Gains and losses in a radio chain multiply. An amplifier that doubles power followed by a cable that halves it gives a net factor of one, but you have to multiply to see that. On a logarithmic scale multiplication becomes addition: +3 dB then −3 dB is plainly zero. A full link budget — transmitter power, feeder loss, antenna gain, path loss, receiver antenna gain — becomes a column of numbers you add up, and that is precisely what the gain field on this page demonstrates in miniature.
The useful mental shortcuts follow from the same definition. Three decibels is a factor of two in power, near enough. Ten decibels is a factor of ten exactly. Twenty decibels is a factor of a hundred. Once those three are in your head you can estimate most conversions before you reach for a calculator, and use this page to confirm rather than to discover.
dBm, dB and dBW Are Three Different Things
This is the distinction worth being pedantic about. A plain dB figure is dimensionless — it is a ratio between two powers and describes a change, a gain, a loss or a difference. It cannot be converted to watts, because there is nothing to convert; asking how many watts are in 10 dB is like asking how many metres are in doubling. If a number is genuinely a ratio, the decibel calculator is the tool for it, not this one.
dBm and dBW are absolute powers, because each has a stated reference baked in. They convert to watts unambiguously, and this page does it. The suffix is doing real work: strip it off and you have destroyed the information that made the figure absolute. A specification that says "output 30" is meaningless; "output 30 dBm" is one watt.
There is a wider family of these referenced decibel units, and none of them are interchangeable. Decibels relative to one microvolt, decibels relative to a carrier, decibels relative to full scale in a digital audio system, and the acoustic decibel referenced to a sound pressure of twenty micropascals are all separate scales with separate references. The acoustic ones in particular have nothing to do with this page — for those the sound level converter and the noise exposure calculator cover the ground, and mixing an acoustic dB figure into an RF calculation produces a number with no physical meaning.
Where the Voltage Figure Is and Is Not Valid
Converting power to voltage requires an impedance, and it requires the load to actually be at that impedance. The relation is VRMS = √(P × Z), which is the standard power law rearranged. At 50 Ω, 0 dBm works out to 0.2236 V RMS, a figure worth remembering because it turns up constantly on spectrum analyser and signal generator front panels.
Where it stops being valid is any circuit that is not matched. If your antenna presents 35 ohms with a reactive component, the voltage at the feed point is not what this field reports, and a standing wave on the feeder means the voltage varies along the cable. The power figures remain correct regardless — power is power — but treat the voltage as a nominal test-equipment figure rather than a measurement of your circuit. For general circuit work the Ohm's law calculator and the electrical power calculator handle voltage, current and resistance directly.
Reading Real Specifications Without Getting Caught
Datasheets are inconsistent about whether a quoted figure is average power, peak power or peak envelope power, and dBm alone does not say which. A pulsed radar with a one per cent duty cycle has a peak power a hundred times its average, which is 20 dB apart — a gap large enough to make a specification look either impressive or broken depending on which one the writer meant. If the document does not say, the figure is ambiguous and no converter can resolve it for you.
The second trap is EIRP. Regulatory limits on transmitters are frequently written as effective isotropic radiated power, which is conducted power plus antenna gain. A transmitter rated at 20 dBm feeding a 6 dBi antenna radiates 26 dBm EIRP, and it is the EIRP figure that a regulator cares about. Enter the antenna gain in the gain field on this page and you have the conversion; what you do not have is legal advice about any specific limit, which depends on your country, band and licence class.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Treating a plain dB figure as a power — dB is a ratio with no reference and cannot become watts. Only dBm, dBW and other referenced units can.
- Using 20 log instead of 10 log — the factor of twenty applies to amplitude and voltage ratios, not to power. dBm is a power unit and always takes ten.
- Adding dBm figures together — two transmitters at 30 dBm do not make 60 dBm. Adding logarithmic absolute powers is meaningless; convert to watts, add, and convert back.
- Forgetting the sign on losses — cable and path loss must be entered as negative gain. Entering a positive number turns a lossy feeder into an amplifier.
- Quoting a voltage without an impedance — the volts figure here is meaningless unless the load really is at the impedance you entered and the system is matched.
Related Free Tools From Arb Digital
For ratio arithmetic rather than absolute power, the decibel calculator handles the 10-log and 20-log cases and the addition of multiple sources. Linear power units go through the power converter, and general energy work through the energy converter. Circuit-side calculations are covered by the Ohm's law calculator, the electrical power calculator and the voltage divider calculator. If your problem is the wave rather than the power, the frequency converter and the wavelength calculator connect frequency to wavelength. The full free online tools hub lists everything Arb Digital publishes.
Frequently Asked Questions
Exactly one watt. Thirty divided by ten is three, ten to the third power is one thousand, and the units are milliwatts, so the answer is one thousand milliwatts or one watt. This is the anchor point most people memorise first.
Only the reference power. dBm is measured against one milliwatt, dBW against one watt. Because a watt is a thousand milliwatts, every dBm figure is exactly thirty larger than the same power in dBW, with no exceptions.
No. A plain dB figure is a ratio between two powers with no fixed reference, so it describes a change rather than an amount. Only referenced units such as dBm and dBW correspond to an absolute number of watts.
Because the power is below one milliwatt. Negative dBm is completely normal and describes most received signals. A mobile phone receiving at minus ninety dBm is picking up one thousandth of a millionth of a milliwatt, and that is a perfectly healthy signal.
Ten, for power. The factor of twenty applies when the ratio is of amplitudes such as voltage or sound pressure, because power goes as the square of amplitude. dBm is defined as a power ratio, so it always uses ten.
Convert each to watts, add the watts, then convert the total back to dBm. Adding the dBm numbers directly is wrong. Two sources at thirty dBm total two watts, which is thirty-three dBm, not sixty.
Because power alone does not fix a voltage. The same power produces different voltages across different loads, following the square root of power times impedance. Fifty ohms is the RF convention and seventy-five ohms the video convention, so the field defaults to fifty and is editable.
This tool is provided for educational and engineering study use. It performs the published dBm conversion and does not account for duty cycle, mismatch, measurement bandwidth or regulatory definitions, so treat its output as arithmetic rather than a compliance measurement.
