Decibel converter
Pick a mode, enter a value and see it in every related form. Ratios convert dB, power ratio, voltage ratio and nepers; power levels convert dBm, dBW, mW and W; voltage levels convert dBu, dBV and volts.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose a mode: ratio, power level or voltage level.
- Choose the unit your value is in and type the value. A dot or a comma works as the decimal separator.
- Read the same quantity in the other forms below.
- Power ratio = 10^(dB / 10); voltage ratio = 10^(dB / 20).
- dBm = 10·log10(P / 1 mW); dBW = dBm − 30.
- dBV = 20·log10(V / 1 V); dBu = 20·log10(V / 0.7745967 V), so dBu = dBV + 2.2185.
- 1 Np = 20 / ln 10 dB = 8.685890 dB.
The dBu reference is √(0.001 W × 600 Ω) = √0.6 V.
Worked example
A gain of 3 dB is a power ratio of 10^0.3 = 1.995 (about 2×) and a voltage ratio of 10^0.15 = 1.413. Professional audio at +4 dBu is 0.7746 × 10^(4/20) = 1.228 V rms, which is +1.78 dBV.
Tips
- Add decibels to chain gains and losses; multiply the ratios instead and you get the same result.
- A 3 dB change is roughly double or half the power; 6 dB is roughly double the voltage. These are rules of thumb, the exact values are shown above.
Frequently asked questions
What is the difference between 10·log and 20·log?
Power is converted with 10·log10 of the ratio, while voltage, current and sound pressure (field quantities) use 20·log10, because power goes with the square of voltage. A power ratio of 100 and a voltage ratio of 10 are both 20 dB.
What are the references for dBm, dBW, dBV and dBu?
dBm is relative to 1 mW and dBW to 1 W, so 0 dBm = −30 dBW. dBV is relative to 1 V rms. dBu is relative to 0.7746 V rms, the voltage that gives 1 mW in 600 Ω, so 0 dBu = −2.218 dBV.
How many decibels is one neper?
1 Np = 20 / ln 10 dB ≈ 8.6859 dB for amplitude quantities, and 1 dB ≈ 0.11513 Np. Nepers use the natural logarithm.
Why can I not enter zero or a negative ratio?
The logarithm of zero or of a negative number does not exist. A power or voltage ratio must be positive. A negative dB value is fine and means a loss.
Is dBu the same as dBm?
Only on a 600 Ω load. Then 0 dBu equals 0 dBm. In general dBm is a power level and dBu a voltage level, so they are different quantities.