dBm, dBV and dBu
The decibel on its own is a ratio — it tells you how much bigger or smaller one signal is compared to another. But sometimes you need to express an absolute level, not just a ratio. That is where dBm and dBW come in for power, and dBV and dBu for voltage. You will see all four on datasheets, audio equipment and test instruments, and this lesson shows you how to read and convert each one.
- Why we need absolute decibel units
- dBm — decibels relative to 1 milliwatt
- Key dBm values to memorise
- Converting between watts and dBm
- dBW — decibels relative to 1 watt
- dBm / dBW calculator
- dBV — decibels relative to 1 volt
- dBu — decibels relative to 0.775 volt
- Converting between dBV and dBu
- When each unit is used
Why we need absolute decibel units
Suppose a datasheet says an amplifier has 20 dB of gain. That is useful — it means the output power is 100 times the input power. But it does not tell you the actual power levels. For that, you need a reference point. If the input is −10 dBm, you know the output is +10 dBm. Without the absolute unit, you can only describe ratios, not levels.
For power, two absolute references are in everyday use:
- dBm — decibels relative to 1 milliwatt. Used for signal levels on datasheets and test instruments, wireless module output power, and small-signal measurements.
- dBW — decibels relative to 1 watt. Used for high-power systems such as large amplifiers, broadcast transmitters and satellite links.
dBm — decibels relative to 1 milliwatt
The formula for converting a power in milliwatts to dBm is:
which simplifies to:
P(dBm) = 10 × log10(PmW)
To go the other way — from dBm back to milliwatts:
The reference point, 0 dBm, is exactly 1 milliwatt. Values above 0 dBm represent more than 1 mW; values below 0 dBm represent less.
Key dBm values to memorise
Just as with plain decibels, a handful of reference points get you through most calculations without needing a calculator:
| dBm | Power in milliwatts | Power in watts | Everyday example |
|---|---|---|---|
| −120 dBm | 0.000 000 001 mW | 1 pW (picowatt) | Noise floor of a sensitive measuring instrument |
| −90 dBm | 0.000 000 001 mW × 1000 | 1 pW × 1000 = 1 nW | Weakest usable Wi-Fi signal |
| −60 dBm | 0.000 001 mW | 1 nW × 1000 = 1 μW | Typical received Wi-Fi signal near the router |
| −30 dBm | 0.001 mW | 1 μW | Small signal at a sensor input |
| 0 dBm | 1 mW | 0.001 W | Reference point; signal generator output |
| 10 dBm | 10 mW | 0.01 W | Bluetooth module (Class 1) output |
| 20 dBm | 100 mW | 0.1 W | Wi-Fi router output |
| 30 dBm | 1000 mW | 1 W | Small speaker amplifier |
| 40 dBm | 10 000 mW | 10 W | Desktop speaker amplifier |
| 50 dBm | 100 000 mW | 100 W | Hi-fi or car audio amplifier |
| 60 dBm | 1 000 000 mW | 1 kW | Large PA amplifier or microwave oven magnetron |
Notice the pattern: every +10 dBm multiplies the power by 10, and every +3 dBm approximately doubles it (because 100.3 ≈ 2). These two facts alone let you work out most dBm values mentally once you know a single reference point.
The dBm scale: every 10 dB is a factor of ten in power, and 0 dBm is 1 mW. Wireless modules, signal generators and RF amplifiers all specify power in dBm.
View LargerConverting between watts and dBm
The conversion steps are:
- Convert the power to milliwatts (multiply watts by 1000).
- Take log10 of the milliwatt value.
- Multiply by 10.
100 W = 100 000 mW
log10(100 000) = 5
P(dBm) = 10 × 5 = 50 dBm
PmW = 10(−110/10) = 10−11 = 0.000 000 000 01 mW
= 10 picowatts (pW)
dBW — decibels relative to 1 watt
dBW uses exactly the same formula as dBm, but the reference is 1 watt instead of 1 milliwatt:
which simplifies to:
P(dBW) = 10 × log10(PW)
The key relationship between dBm and dBW is straightforward: since 1 watt = 1000 mW = 30 dBm, every dBW value is exactly 30 dB higher in milliwatt terms:
or equivalently:
dBm = dBW + 30
Example: 1 kW = 60 dBm = 60 − 30 = 30 dBW
Watts / dBm / dBW Calculator
Watts to dBm and dBW
Enter a power in watts to convert it to dBm and dBW.
dBm to Watts and dBW
Enter a power level in dBm to convert it to milliwatts, watts and dBW.
dBV — decibels relative to 1 volt
dBm and dBW describe power, which depends on the load the signal is driving. In audio and measurement work you usually care about the voltage of a signal, measured across a high-impedance input. For voltage levels, two references are in everyday use, and both use the 20 × log formula because they compare voltages:
VRMS = 10(dBV / 20) volts
The reference, 0 dBV, is exactly 1 V RMS. dBV is the unit used for consumer audio equipment and for the vertical scale of many oscilloscope FFT displays and audio analyzers.
V = 10(−10/20) = 10−0.5 = 0.316 V RMS
dBu — decibels relative to 0.775 volt
dBu uses a reference of 0.775 V RMS. That odd-looking number is the voltage that delivers exactly 1 mW into a 600 Ω load (V = √(0.001 × 600) = 0.775 V), the standard impedance of old telephone and studio equipment. The 600 Ω load is no longer required, but the reference voltage stuck.
Professional audio equipment (mixing desks, studio interfaces, PA systems) uses dBu. Professional line level is nominally +4 dBu = 0.775 × 10(4/20) ≈ 1.23 V RMS.
Converting between dBV and dBu
Since 1 V is 2.2 dB above 0.775 V (20 × log(1 / 0.775) = 2.21 dB), the conversion is simply:
dBV = dBu − 2.2
When each unit is used
| Unit | What it measures | Typical use |
|---|---|---|
| dBm | Absolute power (ref: 1 mW) | Signal levels on datasheets and instruments, wireless module output power, spectrum analyzer readings |
| dBW | Absolute power (ref: 1 W) | High-power systems, broadcast and satellite link budgets |
| dBV | Absolute voltage (ref: 1 V RMS) | Consumer audio, microphone sensitivity, audio analyzer and oscilloscope FFT displays |
| dBu | Absolute voltage (ref: 0.775 V RMS) | Professional audio, mixing desks and studio equipment |
In a real system you will often mix these units. For example: a microphone's sensitivity in dBV, a preamplifier's gain in dB, a mixer's output in dBu, and a power amplifier's output in dBm or watts. As long as you keep track of what each number references, the arithmetic stays simple.
Frequently Asked Questions
What does a microphone sensitivity of −40 dBV mean?
Microphone sensitivity is the output voltage for a standard sound level (94 dB SPL, about the level of loud speech close up). −40 dBV means the microphone produces 10(−40/20) = 0.01 V, or 10 mV RMS, at that level. A microphone rated at −50 dBV produces only 3.2 mV and needs 10 dB more preamplifier gain to reach the same output.
Why do some specifications use negative dBm numbers?
Because the signal levels involved are far below 1 milliwatt. A Wi-Fi receiver might work down to −90 dBm, which is 10−9 milliwatts, or one picowatt — an inconveniently small number to write out. The negative dBm value is much easier to work with and makes comparisons simple: −90 dBm is 10 dB (ten times) weaker than −80 dBm.
Can I convert dBm to dBV directly?
Only if you know the impedance, because power depends on both voltage and resistance (P = V²/R). In a 50 Ω system, used by most RF test equipment, 0 dBm = 0.224 V RMS = −13 dBV. In a 600 Ω system, 0 dBm = 0.775 V = 0 dBu, which is where the dBu reference came from. Always check the impedance before converting between power and voltage units.
Why does my audio interface have a switch for −10 dBV and +4 dBu?
The switch matches the interface to the two common line levels. Consumer equipment works around −10 dBV (0.316 V) and professional equipment around +4 dBu (1.23 V), a difference of about 12 dB. Setting the switch correctly keeps the signal well above the noise without overloading the input.
Test Your Knowledge
Answer the questions below to check your understanding of this lesson. Every answer can be found in the lesson above.