Unit Converters
Decibel (dB) Calculator
Decibel (dB) Calculator converts between a plain ratio and its decibel equivalent, for both power ratios and amplitude (voltage or sound-pressure) ratios.
Power ratios use 10·log10(ratio); amplitude ratios use 20·log10(ratio) -- pick the quantity type that matches what you're measuring.
How to Use the Decibel (dB) Calculator
- Open the Decibel Calculator tool.
- Choose whether you're working with a power ratio or an amplitude/voltage ratio.
- Choose the conversion direction: ratio to dB, or dB to ratio.
- Enter your value.
- Read the converted result, updated instantly.
Frequently Asked Questions
When do I use the power formula versus the amplitude formula?
Use the power formula (10·log10) for quantities like energy, wattage or intensity. Use the amplitude formula (20·log10) for quantities like voltage, current or sound pressure.
Why is the amplitude formula multiplied by 20 instead of 10?
Power is proportional to the square of amplitude, so converting an amplitude ratio to decibels effectively doubles the exponent, which works out to a factor of 20 instead of 10 in the logarithmic formula.
What does a negative dB value mean?
A negative decibel value means the ratio is less than 1 -- i.e. a reduction or loss rather than a gain.