Conversion calculator

dBm to Watts Converter

Absolute decibel scales turned into watts and volts — and the two that cannot be, listed with the reason rather than left out of the menu.

Level

Scale

Convert to

Try:

30 dBm

1.000 W

0.00 dBW · Power relative to one milliwatt. The RF and telecoms standard.

Every scale, and what it references

The suffix is the reference — it is not decoration
Scale	Reference	Multiplier	Your 30 dB is
dB (plain)	none	—	not a level
dBm	1.000 mW	10·log₁₀	1.000 W
dBW	1.000 W	10·log₁₀	1.000 kW
dBu	774.597 mV	20·log₁₀	24.495 V
dBV	1.000 V	20·log₁₀	31.623 V
dBFS	none	20·log₁₀	not a level

The last column is the argument. The same number of decibels is a milliwatt, a watt, three quarters of a volt or nothing at all, depending entirely on what follows the “dB”. A decibel on its own is a ratio between two things, and the suffix is what supplies the second thing.

The audio gap that is not 14 dB

dBu reference0.7746 Vthe voltage that gave 1 mW into 600 Ω

dBV reference1.0000 Vexactly one volt

So the two scales sit apart by2.218 dBnot zero, and not a rounding

+4 dBu against −10 dBV11.78 dBand not the 14 the numbers suggest

Professional gear runs at +4 dBu and consumer gear at −10 dBV, and subtracting one from the other gives 14 — which is wrong, because they are measured against different references. The real difference is 11.8 dB. That is why a consumer output into a professional input sounds quiet and why the interface boxes that fix it exist.

Ten log or twenty log

The same decibels, as a power ratio and as an amplitude ratio
Decibels	Power ratio	Voltage or pressure ratio
1 dB	1.259	1.122
3 dB	1.995	1.413
6 dB	3.981	1.995
10 dB	10.000	3.162
20 dB	100.000	10.000
30 dB	1.00e+3	31.623
40 dB	1.00e+4	100.000
60 dB	1.00e+6	1.00e+3

Power ratios use 10·log₁₀ and voltage or pressure ratios use 20·log₁₀, because power goes as the square of amplitude. So 3 dB doubles power and 6 dB doubles voltage, and using one multiplier where the other belongs doubles or halves every figure while producing an answer that looks perfectly reasonable.

Something to measure against

Radio power levels, across 254 decibels
Where	dBm	In watts	Note
Thermal noise floor, 1 Hz bandwidth	-174	3.981e-21 W	The physical limit at room temperature. Everything else is measured against it.
GPS signal at the antenna	-130	1.000e-16 W	Below the noise floor of its own bandwidth, and recovered by correlation.
Usable mobile signal	-100	1.000e-13 W	Four bars is nearer −70; −110 is where calls start dropping.
Wi-Fi at the far end of a house	-70	1.000e-10 W	Still workable. −80 is not.
Wi-Fi beside the router	-30	1.000 µW	A hundred thousand times the power of the −80 case.
Wi-Fi transmitter output	20	100.000 mW	100 mW, the common regulatory ceiling.
Mobile handset at full power	33	1.995 W	2 W. Rare, and hard on the battery.
FM broadcast transmitter	80	100.000 kW	100 kW. Thirteen orders of magnitude above the noise floor.

This table is why the decibel exists. From the thermal noise floor to a broadcast transmitter is a factor of about ten to the twenty-fifth, and writing that column in watts would be unreadable — but as decibels it is a span of 254, and the difference between any two rows is a subtraction.

What this converter covers

dBm, dBW, dBu and dBV to watts and volts, conversions between them, the load impedance where one is needed, and a radio ladder spanning 254 decibels.

dBm and dBW to watts, in either direction
dBu and dBV to volts, and the 2.218 dB between the two references
Crossing between voltage and power scales through a load impedance
Ten-log against twenty-log ratios, side by side
A plain dB and dBFS, listed with the reason they have no absolute value

The suffix is the reference +4 and −10 are 11.8 apart 10·log and 20·log dBm · dBW · dBu · dBV

Convert a value Why +4 and −10 are 11.8 apart

Free, no signup — exact by definition, not an estimate.

Updated 8 September 2026

At a glance

Formula shown: value = reference × 10^(dB ÷ multiplier), with 10 for power and 20 for amplitude
Scenario support: 30 dBm is exactly 1 W · 0 dBm into 600 Ω is exactly 0 dBu
Educational estimate: Planning support from the values you enter — not professional advice.

The suffix is the definition

A decibel is not an amount of anything. It is a ratio between two quantities, expressed logarithmically, and on its own it carries no value at all.

“The amplifier has 20 dB of gain” is complete — it is a comparison of output to input. “The signal is 20 dB” is not; it is missing the other half of the comparison.

The suffix supplies it, and every suffix supplies a different one:

The same number, six different meanings
Written	Reference	0 on that scale is
dB	none	nothing — it is a ratio of 1
dBm	1 milliwatt	1 mW
dBW	1 watt	1 W
dBu	0.7746 volts	0.7746 V
dBV	1 volt	1 V
dBFS	digital full scale	the ceiling, not a voltage

This is why the calculator above refuses a plain dB and refuses dBFS, and lists both in the menu with the reason attached. An omission looks like an oversight; a stated refusal is the answer.

Ten log and twenty log

The second thing hidden inside a decibel figure is which multiplier produced it, and getting it wrong doubles or halves everything without looking wrong.

For power — watts, and therefore dBm and dBW — the formula is 10·log₁₀ of the ratio. For amplitude — volts, and therefore dBu and dBV, and sound pressure too — it is 20·log₁₀.

The reason is that power goes as the square of amplitude. Double the voltage and you quadruple the power, so the same physical change is 6 dB either way only because the multiplier compensates.

Which produces the two facts worth memorising, and the one that trips people up when they are confused: 3 dB doubles power and 6 dB doubles voltage. Ten decibels is ten times the power and about 3.16 times the voltage; twenty decibels is a hundred times the power and ten times the voltage.

The table in the calculator prints both columns side by side deliberately. If a figure from a datasheet does not behave the way you expect, checking which column it came from is usually the answer.

Why +4 and −10 are 11.8 apart

Professional audio equipment runs at a nominal +4 dBu. Consumer equipment runs at −10 dBV. The obvious subtraction gives 14 dB, and the obvious subtraction is wrong.

The two figures are measured against different references. dBu references 0.7746 V and dBV references exactly 1 V, so the scales themselves sit 2.218 dB apart before any signal is involved.

Work it in volts and it comes out cleanly:

+4 dBu is 0.7746 × 10^0.2, which is 1.228 V. −10 dBV is 10^−0.5, which is 0.316 V. The ratio is 3.88, and 20·log₁₀ of that is 11.78 dB.

Two decibels is not much on its own, but the direction matters: a consumer output feeding a professional input is quieter than expected, so the gain has to come from somewhere, and taking it from a preamp raises the noise floor along with the signal. That is the entire market for the little level-matching boxes.

The 0.7746 has its own history: it is √0.6, the voltage that delivers exactly one milliwatt into 600 ohms. Which means 0 dBm into 600 Ω is exactly 0 dBu — the calculator above will confirm it, and it is the seam where the audio and telephony worlds were once the same world.

Crossing needs an impedance

dBm to dBW is arithmetic — both are power, and the answer is always exactly 30 dB. dBu to dBV is arithmetic too — both are voltage, and the answer is always 2.218 dB.

Going from one family to the other is not arithmetic. Power is voltage squared divided by impedance, so you cannot get from a voltage to a watt without knowing what the voltage is driving.

So the calculator asks for a load impedance, and only when it needs one. Without it, it returns nothing — which is the honest answer rather than a missing feature.

In radio the impedance is nearly always 50 ohms and the question rarely arises. In audio it once was 600 ohms and now generally is not: modern equipment drives a high-impedance input with a low-impedance output, so almost no power flows and a power figure for an audio line level is a number without a use.

Which is worth stating plainly, because the arithmetic will happily produce one. Converting +4 dBu into milliwatts is a legitimate calculation and, for almost any modern audio interconnect, a meaningless one.

dBFS points the other way

Digital audio uses its own scale, and it is the only one here where zero is the top rather than a reference in the middle.

dBFS is decibels relative to full scale — the largest number the format can represent. So 0 dBFS is the ceiling, everything real is negative, and a recording peaking at −6 dBFS is using half the available voltage swing.

There is no absolute value behind it. What voltage 0 dBFS corresponds to depends entirely on the converter and how the equipment around it was calibrated, and studios and broadcasters use different alignments: one facility may align 0 VU to −20 dBFS and another to −18, and both are correct within their own reference.

Which is why the calculator refuses to give dBFS a voltage. The number describes a position within a digital range and carries no information about what the analogue world does with it.

The practical consequence is that headroom in digital is unlike headroom in analogue. Above 0 dBFS there is nothing at all — no gentle saturation, just clipping — which is why digital recording levels sit far lower than tape levels ever did, and why a meter reading −18 dBFS is not a quiet recording.

Sources and methodology

The first two establish that a decibel is a ratio and that a level requires a stated reference; the third and fourth are where the specific audio and radio references on this page come from.

Guide for the Use of the International System of Units (NIST SP 811)National Institute of Standards and Technology · verified 2026-09-08 · The treatment of the bel and the decibel as units for logarithmic ratio quantities, and the requirement that the reference quantity be stated for any level to have meaning
IEC standards — letter symbols for logarithmic quantitiesInternational Electrotechnical Commission · verified 2026-09-08 · IEC 60027-3, which defines logarithmic quantities and their units and specifies how a reference level is indicated, and the resulting distinction between a ratio in decibels and a level such as dBm
Audio Engineering SocietyAES · verified 2026-09-08 · The dBu and dBV reference levels used in professional and consumer audio and the nominal +4 dBu and −10 dBV operating levels, which are the source of the 11.8 dB difference described on this page
International Telecommunication UnionITU · verified 2026-09-08 · The use of dBm and dBW as absolute power levels in radio and telecommunication recommendations, including the thermal noise floor reference that anchors receiver sensitivity figures

Conversion note

These are unit conversions between defined reference levels, and they say nothing about what a real system will do. Real levels depend on gain structure, on headroom, on source and load impedances that are rarely the nominal figures, and on measurement bandwidth and weighting — a figure in dBm is only meaningful alongside the bandwidth it was measured in, and an audio level alongside whether it is peak, RMS or averaged. Cable losses, connector losses, standing waves and antenna gains all move a radio figure and none of them is here. The 600-ohm impedance behind the dBu reference is historical: modern audio equipment is almost always voltage-driven into a high-impedance input, so converting an audio level to a power figure is usually meaningless even though the arithmetic will produce one. Sound pressure levels are a separate scale again with their own reference and weighting curves, and are not covered here. For anything that has to meet a specification, measure it.

How we calculate · Found an error? email us

Authorship & verification

Written and maintained by Jay Sudha, a business operator who builds spreadsheet-based calculators.

Formula and examples verified on 8 September 2026
Self-reviewed — written and checked by the same person, with no independent reviewer
Educational estimate only
Formula tested against 86 automated cases · last run 2026-09-08
How calculators are verified

What's changed (9 updates)

Published 8 September 2026

Published the dBm to Watts Converter: dBm, dBW, dBu and dBV to watts and volts and between each other, with a load impedance where one is genuinely needed.
States the thing the question usually omits: a decibel is a RATIO and has no value of its own, so the suffix is not decoration but the whole definition.
Lists the plain dB and dBFS in the scale menu and refuses both with the reason, rather than leaving them out -- a bare decibel has no reference, and what 0 dBFS corresponds to depends on a converter's calibration.
Separates the two multipliers. Power ratios are ten log and amplitude ratios are twenty log, so three decibels doubles power and six doubles voltage, and using the wrong one halves or doubles every figure while looking reasonable.
Corrects the arithmetic everyone does on audio levels: +4 dBu against -10 dBV is 11.78 dB and not 14, because the two scales reference 0.7746 V and 1 V respectively and sit 2.218 dB apart before any signal is involved.
Records the historical seam that explains the odd 0.7746: it is the square root of 0.6, the voltage that delivers exactly one milliwatt into 600 ohms, so 0 dBm into 600 ohms is exactly 0 dBu.
Requires a load impedance to cross between voltage and power scales and refuses without one, and notes that for modern voltage-driven audio interconnects a power figure is arithmetically available and practically meaningless.
Gives a radio ladder spanning 254 decibels from the thermal noise floor to a broadcast transmitter, which is the reason the unit exists.
Verified by 86 automated cases, asserting that each referenceless scale returns null and carries a stated reason, that 0 dBm into 600 ohms is exactly 0 dBu, that the professional-consumer gap is 11.78 rather than 14, and that the power ratio is the amplitude ratio squared at every level.

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