dBm to Watts Calculator (dBm, dBW, dBu and Volts)

Background.

dBm is a power, written on a logarithmic scale. That one sentence separates it from the decibel figures that describe gain, and it is the distinction most people trip over. A gain of 20 dB is a ratio — it multiplies whatever arrives by a hundred and tells you nothing about how much that is. A level of 20 dBm is an actual quantity of power: 100 milliwatts, no context required. The letter on the end is the reference, and without a reference a decibel figure cannot name a power at all.

ITU-R V.574-5, the recommendation that governs how the decibel is used in telecommunications, sets it out in §6.1: when the reference power is one watt the level is written dBW, and when it is one milliwatt it is written dBm. Everything else follows from that. Zero dBm is one milliwatt. Thirty dBm is one watt, because a factor of a thousand is thirty decibels. Minus thirty dBm is one microwatt, and every further thirty decibels down is another factor of a thousand. Once you have those three anchors you can read most of the scale without a calculator.

What you cannot do without one is turn a level into volts, and that is the second half of what this page does. A power level does not by itself specify a voltage — the same milliwatt is 0.224 V in a 50 Ω system and 0.775 V in a 600 Ω one. The impedance is therefore an editable field rather than a hidden constant, because there is no universal value: 50 Ω is the radio-frequency convention, 75 Ω belongs to video and antenna feeds, 600 Ω to legacy line-level audio, and 4 to 8 Ω to a loudspeaker. Changing that field changes the volts and amps this page reports; it does not change the power level, and the page says so.

A fifth mode handles dBu, which is where audio and radio meet and misunderstand each other. dBu is a voltage level, not a power level, referred to 0.775 V RMS — a figure ITU-R V.574-5 §6.5 records as "generally adopted" because it dissipates one milliwatt in 600 ohms. It is also very slightly rounded: the exact voltage is the square root of 0.6, which is 0.774597 V, so a dBu figure sits 0.0045 dB below the dBm figure it is supposed to equal at 600 Ω. This page uses the standard's own 0.775 V and reports that offset rather than quietly correcting it, because 0.775 V is what every audio interface in the world is calibrated against.

One notational disagreement is worth knowing about, and it is recorded on the page rather than smoothed over. ITU-R V.574-5 §4.2 endorses "dBm" as accepted usage, calling it a simplified symbol for dB(mW). NIST Special Publication 811 §8.7 states the opposite: that the rules of IEC 60027-3 preclude the symbol dBm, because unit symbols may not carry attachments, and that the correct form is a level with its reference stated separately. Both are right within their own rules. This calculator follows the ITU, because dBm is what every datasheet, spectrum analyser and link budget actually prints — but if you are writing for a metrology audience, write the level and its reference instead.

Two scope limits, stated here rather than in a collapsed FAQ. The voltage and current outputs assume a purely resistive impedance; a reactive load carries current that delivers no power, and this page does not model that. And a power level on its own says nothing about whether a signal is usable — at very low levels the number only means something alongside a bandwidth and a noise figure, because thermal noise alone is about −174 dBm in every hertz at room temperature.

What is dbm to watts calculator?

A level is a logarithm of a ratio between a quantity and a fixed reference of the same kind. Absolute power level, defined in ITU-R V.574-5 §6.1, is the ratio between the power at a point in a transmission channel and a specified reference power, expressed in decibels. Name the reference and the level names a power.

The two references in universal use are one milliwatt, giving dBm, and one watt, giving dBW. They differ by exactly 30 dB in every circumstance, because a watt is a thousand milliwatts. Satellite and broadcast work tends to use dBW because the powers are large; test equipment, radio modules and everything to do with receivers use dBm because the powers are small and would otherwise be written with a great many leading zeros.

The reason to use a level at all is dynamic range. A receiver might need to work with signals from −10 dBm down to −110 dBm. In watts that is 100 microwatts down to 10 femtowatts — a span of ten billion — and comparing those two numbers on a page is unpleasant. As levels they are a hundred decibels apart, and a hundred is a number people can hold in their heads. Gains and losses in the chain then add and subtract as plain arithmetic on top of the level, which is why the entire practice of link budgeting is written this way.

dBu belongs to a related but different family: it is an absolute voltage level, referred to 0.775 V RMS rather than to a power. ITU-R V.574-5 §6.5 gives the relationship to dBm as Lu = Lp + 10·lg(R/600), which is another way of saying that a voltage level and a power level coincide only in a 600 ohm system. Outside 600 ohms they diverge by ten times the logarithm of the impedance ratio — the same equal-impedance caveat that governs voltage gains in decibels, wearing a different hat.

How to use this calculator.

1. 01Pick the form your number is already in. All five modes return all the others, so there is no separate 'watts to dBm' page to look for — that is this one, with a different first field.
2. 02Type the value. A negative dBm is completely normal and simply means the power is below a milliwatt; almost every received signal in existence is negative in dBm.
3. 03Set the system impedance to what your equipment actually uses before reading the voltage. 50 Ω is the default because most RF work uses it, but a 75 Ω antenna feed or a 600 Ω audio line will give a different voltage for the same power.
4. 04Read the power in whichever unit suits its size. The summary picks one for you — watts, milliwatts, microwatts, nanowatts or picowatts — so you are not counting zeros.
5. 05Use the dBW output when you are working with a link budget that is written in dBW. It is always exactly 30 below the dBm figure, so it is a shift rather than a computation, but having both stops sign errors.
6. 06In audio, use the dBu mode. Enter +4 dBu and you get the 1.228 V RMS that a professional line output is calibrated to, together with what that is in dBm for the impedance you specified.
7. 07To apply a gain or a loss, do it in decibels and add: −70 dBm at the antenna plus 40 dB of amplification is −30 dBm at the receiver. Convert to watts only at the end, when you actually need a physical quantity.

The formula.

Going from a level to a power is the definition read backwards. A level in dBm is 10·lg of the power in milliwatts, so the power in milliwatts is ten raised to the level divided by ten. −70 dBm is 10^(−7) mW, which is 0.0000001 mW, which is 1e-10 W, which is 100 picowatts. Dividing by a thousand converts to watts, and that division is the only difference between the dBm and dBW scales: 10·lg(1000) is exactly 30, so dBW is dBm minus 30 with no rounding anywhere.

The voltage comes from P = V²/Z, rearranged to V = the square root of P·Z. In a 50 Ω system, −70 dBm is the square root of 1e-10 × 50, which is 70.71 microvolts. The current is the square root of P/Z, which is 1.414 microamps; multiply the two together and you recover 1e-10 W, which is the arithmetic check the page's own tests run at three different impedances. Both figures are RMS and both assume the impedance is purely resistive.

The dBu path runs the other way. A dBu figure is a voltage level, so the voltage comes first: 0.775 V times ten raised to the level divided by twenty. The power then follows from V²/Z, which is why the impedance field is not optional in this mode. Because 0.775 V is the rounded value ITU-R V.574-5 §6.5 records as generally adopted rather than the exact square root of 0.6, a dBu figure converted to dBm at 600 Ω lands 0.0045 dB high — 0 dBu is 0.00452 dBm, not 0.00000 dBm. That offset is constant at every impedance and this page reports it rather than removing it, because 0.775 V is the number real audio equipment is built around.

The magnitude bands in the summary are decades of power, and every edge is a definition rather than a judgement: a factor of a thousand in power is exactly 30 dB, so the edges fall at +60, +30, 0, −30, −60 and −90 dBm. Nothing on this page classifies a signal as good or bad, because signal-quality thresholds are vendor conventions rather than published standards and would have to be guessed.

Rounding is split deliberately. The three level outputs — dBm, dBW, dBu — are rounded once at the return boundary to ten decimal places. The power, voltage and current outputs use twelve significant figures instead, because rounding to ten decimal places would turn a −100 dBm signal, 1e-13 W, into exactly zero, and receive levels of −100 dBm are ordinary rather than exotic. The band classification reads the unrounded level, so a figure that displays as exactly −30.0000000000 but is really a shade below it is classified in the lower decade. All internal arithmetic is exact decimal at forty significant digits.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Recommendation ITU-R V.574-5 (08/2015), "Use of the decibel and the neper in telecommunications", Annex 1. Read directly from the ITU PDF on 2026-07-29. Primary source for this page: §6.1 defines absolute power level and states that the symbol dBW is used when the reference power is one watt and dBm when it is one milliwatt; §4.2 records that "common use may give rise to simplified symbols, such as dBm instead of dB(mW)"; §6 sets out the general level notation and the rule that where no number is shown the number 1 is understood; §6.5 defines the absolute voltage level dBu against "a reference voltage with an r.m.s. of 0.775 volt … which corresponds to a 1 milliwatt power dissipated in a resistance of 600 ohms" and gives Lu = Lp + 10 lg(R/600).
2. [2]NIST Special Publication 811, 2008 edition, "Guide for the Use of the International System of Units (SI)", §8.7 "Logarithmic quantities and units: level, neper, bel". Read directly from the NIST PDF on 2026-07-29. The independent check on the level definition — it gives LP = lg(P/P0) B = 10 lg(P/P0) dB and states that "when reporting values of LF and LP, one must always give the reference level" — and the source of the notational conflict recorded on this page: Note 3 states that "the rules of Ref. [5: IEC 60027-3] preclude, for example, the use of the symbol dBm to indicate a reference level of power of 1 mW", which is the opposite of ITU-R V.574-5 §4.2.
3. [3]IEC 60027-3, "Letter symbols to be used in electrical technology — Part 3: Logarithmic and related quantities, and their units". The standard behind both sources above: ITU-R V.574-5 names it in considering clause (b), and NIST SP 811 §8.7 states that its section is based on it and that its rules are what preclude the dBm symbol. Access: gated — sold by the IEC and not opened for this page; every statement attributed to it here is quoted through NIST SP 811, which was read in full.
4. [4]Recommendation ITU-R V.574-5 §1.1 and §1.2. Source of the distinction this page draws between a level and a ratio: §1.1 defines the bel as the decimal logarithm of a power ratio, and §1.2 warns that use of the decibel across unequal impedances "is not appropriate unless adequate information is given concerning the impedances involved" — the same condition that makes dBu and dBm diverge outside 600 ohms. Retrieved 2026-07-29.