Free Online Decibel Converter
Convert dB to voltage, power, and amplitude ratios.
Decibels to ratio
Positive values are gain; negative values are attenuation.
Amplitude: dB = 20 log₁₀(A₂ / A₁) → ratio = 10dB / 20
Ratio to decibels
For example, 2 means twice the power or twice the amplitude, depending on the type.
Doubling power is about +3.01 dB. Doubling voltage or amplitude is about +6.02 dB. The difference exists because power is proportional to the square of amplitude.
Common decibel values
| Decibels | Power ratio | Amplitude ratio |
|---|---|---|
| −20 dB | 0.01× | 0.1× |
| −10 dB | 0.1× | 0.3162× |
| −6 dB | 0.2512× | 0.5012× |
| −3 dB | 0.5012× | 0.7079× |
| 0 dB | 1× | 1× |
| +3 dB | 1.9953× | 1.4125× |
| +6 dB | 3.9811× | 1.9953× |
| +10 dB | 10× | 3.1623× |
| +20 dB | 100× | 10× |
About the Decibel Converter
The decibel is a logarithmic unit used to compare two quantities. It compresses an enormous range of values into a manageable scale, which is why it appears everywhere in audio, radio, acoustics, and telecommunications. This converter turns a decibel value into power and amplitude ratios, converts a ratio back into decibels, and shows the absolute level for common reference standards such as dBm, dBV, dBu, and sound pressure level. It also displays the formulas so you can reproduce the arithmetic yourself.
Why decibels are logarithmic
Human perception of loudness is roughly logarithmic, and physical systems often span many orders of magnitude. A linear scale would need unwieldy numbers to describe the difference between a whisper and a jet engine. The decibel solves this by taking the logarithm of a ratio and multiplying by ten for power quantities or twenty for amplitude quantities. An increase of 10 dB means ten times the power, while an increase of 20 dB means ten times the amplitude. Negative decibels simply indicate a value smaller than the reference.
Power versus amplitude
Power is proportional to the square of voltage, current, or pressure, so the two families of formulas differ by a factor of two. For power, dB = 10 log₁₀(P₂ / P₁). For voltage, current, amplitude, or sound pressure, dB = 20 log₁₀(A₂ / A₁). Doubling power is approximately +3.01 dB, while doubling voltage is approximately +6.02 dB. Confusing the two is one of the most common mistakes in audio calculations, especially when comparing amplifier output with signal level.
Reference levels
- dBW — decibels relative to one watt.
- dBm — decibels relative to one milliwatt, common in radio and telecommunications.
- dBV — decibels relative to one volt RMS.
- dBu — decibels relative to 0.775 volt RMS, a legacy standard from telephone engineering.
- dB SPL — sound pressure level relative to 20 micropascals, the nominal threshold of human hearing.
A decibel value without a reference is only a ratio. Adding a suffix such as dBm or dBV turns it into an absolute level. This converter uses the selected reference to compute the corresponding absolute quantity.
Practical examples
A signal attenuated by 6 dB retains about half its amplitude and a quarter of its power. A 10 dB increase sounds roughly twice as loud to many listeners, even though it is ten times the power. The dynamic range of a CD is about 96 dB, while human hearing spans more than 120 dB between the threshold of hearing and the threshold of pain. In digital audio, 0 dBFS is the maximum representable level, and mixing engineers routinely work with negative values to preserve headroom.
Frequently asked questions
Why is 3 dB sometimes called double power and sometimes half power?
A +3 dB change is approximately double the power, and a −3 dB change is approximately half. The exact value is 10 log₁₀(2) ≈ 3.0103 dB. Amplitude doubles at about +6.02 dB because power scales with amplitude squared.
Can a ratio be negative?
No. Ratios are positive quantities. A negative decibel value corresponds to a ratio below one, not to a negative ratio. The converter rejects zero and negative ratio inputs.
What is the difference between dB SPL and dBFS?
dB SPL measures physical sound pressure in the air relative to 20 micropascals. dBFS measures digital signal level relative to the maximum code value in a digital system. They are different scales and cannot be converted without calibration.