Decibel (dB) Calculator
Convert between ratios and decibels for power and voltage (amplitude).
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Decibel (dB) Calculator by TheCalcTools
How the Decibel (dB) Calculator works
Enter a ratio to see it in decibels, and a dB value to see it as a ratio.
Power ratios use 10 × log₁₀; voltage and amplitude ratios use 20 × log₁₀ because power is proportional to voltage squared.
A gain of 3 dB is about double the power, and 6 dB is about double the voltage.
The formula
Worked example
Using the default values shown in the calculator:
| Ratio (output ÷ input) | 100 |
|---|---|
| Decibels (dB) | 20 |
- A power ratio of 100 is 10 × log₁₀(100) = 20 dB.
- A voltage ratio of 100 is 20 × log₁₀(100) = 40 dB.
- 20 dB as a power ratio is 100×, and as a voltage ratio 10×.
Result:
| Power ratio in dB | 20.000 |
|---|---|
| Voltage ratio in dB | 40.000 |
| dB to power ratio | 100.0000 |
| dB to voltage ratio | 10.0000 |
Frequently asked questions
Why use decibels?
They turn multiplication into addition, which makes it easy to sum gains and losses along a signal path.
Is dB an absolute unit?
No, it is a ratio. Absolute levels need a reference, as in dBm (1 mW) or dBV (1 V).
Sources and further reading
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.