What the Decibel Converter does
This converter takes a level on any decibel scale - dBm, dBW, dBuW, dBV, dBu, dBmV, dBuV - and gives it on all the others, plus the absolute power and voltage behind it. It also converts a plain ratio into decibels using both the 10 log and 20 log rules, and adds up a chain of gains and losses in dB.
Two things cause most decibel mistakes. The first is using 10 log where 20 log belongs: power ratios use 10 log and voltage or current ratios use 20 log, so the same ratio of 2 is 3.01 dB or 6.02 dB depending on which quantity you have. The second is converting between a power scale and a voltage scale without an impedance. This converter refuses to guess one, because assuming 50 or 600 ohms is how wrong numbers get published.
How to use it
- Enter the level you have and choose its scale. The reference for each is shown in the list, because that is what makes the scale absolute.
- Enter the impedance if you need to cross between power and voltage scales. Without it, those rows say so rather than showing a number.
- Read the table: every scale, its reference, and what your level is worth on it.
- For a plain ratio, enter it in the ratio box and take whichever of the two dB figures matches the quantity you measured.
- For a receiver or transmit chain, list each stage's gain or loss in the chain box; decibels simply add, and the running level in dBm is shown alongside.
Reading the results
A dB on its own is a ratio and has no size. dBm, dBW, dBu and dBV are absolute because each names a reference - 1 mW, 1 W, 0.7746 V and 1 V - so 0 dBm is a real amount of power and 0 dB is not an amount of anything.
3 dB is a factor of two in power; 6 dB is a factor of two in voltage; 10 dB is ten times the power and 20 dB is ten times the voltage. Those four are worth memorising, because almost every other figure can be assembled from them.
The relationship between a voltage ratio and a power ratio only holds when the impedance is the same at both points. A voltage gain measured across different impedances is not the power gain, however the arithmetic looks.
Worked example: 0 dBm into 50 ohms
0 dBm means 1 mW by definition, since dBm references 1 milliwatt. In 50 ohms, P = V^2 / R gives V = sqrt(0.001 x 50) = 0.2236 V RMS, which is 632 mV peak to peak for a sine wave.
On the other scales that is -30 dBW, +30 dBuW, -13.01 dBV, -10.79 dBu and +46.99 dBmV. Note that the dBm-to-dBV conversion is the one that needed the impedance; the dBm-to-dBW one did not, because both are power scales.
In 600 ohms instead, the same 1 mW gives 0.7746 V - which is precisely the definition of 0 dBu, and why dBu and dBm coincide in 600 ohm systems. That historical coincidence is the source of a great deal of confusion in audio, where modern equipment is voltage-driven and the 600 ohm assumption no longer applies.
Formulas and scoring rules
- Power ratio to dB
dB = 10 log10(P1 / P2)Use for watts, milliwatts, or anything proportional to power.- Voltage ratio to dB
dB = 20 log10(V1 / V2)Use for volts or amps. The 20 comes from power being proportional to voltage squared.- dBm to watts
P = 0.001 x 10^(dBm / 10)0 dBm = 1 mW; 30 dBm = 1 W; -30 dBm = 1 uW.- dBm to volts
V = sqrt(10^(dBm/10) x 0.001 x R)Needs the impedance R - there is no answer without it.- References
dBm: 1 mW; dBW: 1 W; dBuW: 1 uW; dBV: 1 V RMS; dBu: 0.7746 V RMS; dBmV: 1 mV; dBuV: 1 uV- Gain chains
total dB = sum of each stage's dBLogarithms add, so gains and losses in dB simply add up.
Where 0.7746 volts comes from
dBu references the voltage that dissipates 1 mW in 600 ohms: sqrt(0.001 x 600) = 0.7746 V. Six hundred ohms was the characteristic impedance of telephone lines, and early audio equipment was genuinely matched to it, so 0 dBu and 0 dBm were the same thing.
Modern audio equipment is not impedance matched - a low-impedance output drives a high-impedance input, and almost no power flows. The voltage reference survived because it is what the meters were calibrated to, which is why professional line level is quoted as +4 dBu (1.228 V) and consumer level as -10 dBV (0.316 V). The u in dBu stands for unloaded, marking exactly this change.
Adding levels, which is not the same as adding decibels
Gains in a chain add in decibels because each stage multiplies the signal. Two uncorrelated signals arriving together do not: their powers add, so two equal noise sources give 3 dB more, not 6 dB and certainly not twice the dB figure.
The practical rule is to convert to linear power, add, and convert back. Two equal sources give +3.01 dB; a source 10 dB below another adds 0.41 dB to it; one 20 dB below adds 0.04 dB and can usually be ignored. This is why a noise budget is dominated by its largest contributor and why chasing the second-largest rarely pays.
Limitations: what the result does not prove
- Crossing between power and voltage scales needs an impedance, and the answer is only correct if that impedance is real and resistive at the frequency concerned.
- All voltage figures are RMS. Peak and peak-to-peak conversions shown assume a sine wave; for other waveforms the crest factor differs.
- The gain chain adds decibels only. It does not compute noise figure, intermodulation or compression, all of which need each stage's own specifications.
- dBA, dBSPL and other acoustic scales are not included: they reference sound pressure and involve frequency weighting, which is a different subject.
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Standards and sources
- Texas Instruments - Op amps for everyone design reference - checked 19 Sep 2026
- Analog Devices - Understanding decibels and their use in RF measurement (MT-016)
- Audio Engineering Society - AES-R2 on standard audio levels and references
Frequently asked questions
How do I convert dBm to watts?
P = 0.001 x 10^(dBm/10). So 0 dBm is 1 mW, 30 dBm is 1 W, and -110 dBm - a typical receiver sensitivity - is 10 femtowatts. Every 10 dB is a factor of ten, and every 3 dB is a factor of two.
When do I use 10 log and when 20 log?
10 log for power ratios and 20 log for voltage or current ratios. The factor of two exists because power is proportional to voltage squared, and squaring inside a logarithm becomes a multiplication by two outside it.
What is the difference between dB and dBm?
dB is a ratio and has no absolute size - it tells you how much bigger or smaller, not how big. dBm is an absolute level referenced to 1 milliwatt, so it names a specific amount of power. Adding 3 dB to 10 dBm gives 13 dBm; adding 3 dBm to 10 dBm is meaningless.
What is 0 dBu in volts?
0.7746 V RMS, the voltage that would dissipate 1 mW in 600 ohms. Professional line level of +4 dBu is 1.228 V, and consumer level of -10 dBV is 0.316 V - which is why connecting the two without a level shift gives either a weak signal or a distorted one.
Why do I need an impedance to convert dBm to dBV?
Because one is a power scale and the other a voltage scale, and power and voltage are related by P = V^2/R. Without R there is no unique answer. This converter says so instead of quietly assuming 50 or 600 ohms.
Do decibels add or multiply?
They add, which is the whole point of using them. A 20 dB gain after a 3 dB loss is 17 dB. In linear terms that is 100 x 0.5 = 50, but you never have to do that multiplication.
How much is 3 dB, really?
A factor of 1.995 in power - near enough double - or 1.41 in voltage. It is the half-power point, which is why filter cutoffs are quoted at -3 dB. In audio it is about the smallest level change most listeners reliably notice.
Can I add two signal levels in dBm by adding the numbers?
No. Convert both to watts, add the powers, then convert back. Two equal uncorrelated signals give 3 dB more than one, not twice the dBm figure. A signal 20 dB below another contributes only 0.04 dB, which is why noise budgets are dominated by their largest term.
Last reviewed by the A2Z.Tools team against the sources listed above.
