Decibel (dB) Calculator

Compute decibels (dB) from an input and output value. Power ratios use 10 log10, while voltage, amplitude, and sound pressure ratios use 20 log10.

Input

Enter the input (reference) value, the output value, and the ratio type to compute decibels (dB).

Ratio type

Power uses 10 log10; voltage, amplitude, and sound pressure use 20 log10.

Input value (reference)

same unit

The reference value used as the denominator of the ratio.

Output value

same unit

The output value, in the same unit as the input value.

Result

Decibels

3.0103dB

Computed as 10 times log10(output / input).

Ratio (output / input)

Type

Power

Power ratios use 10 log10(P2 / P1); voltage, amplitude, and sound pressure ratios use 20 log10(V2 / V1). The coefficient doubles because power is proportional to amplitude squared.

How it works

A decibel (dB) expresses the ratio of two quantities on a base-10 logarithmic scale, measured from a reference (input) value to an output value.
For power ratios: L = 10 times log10(output / input) in dB. A power ratio of 2 gives about 3.0103 dB, and 10 gives 10 dB.
For voltage, amplitude, or sound pressure ratios: L = 20 times log10(output / input) in dB. A voltage ratio of 2 gives about 6.0206 dB, and 10 gives 20 dB.
Because power is proportional to amplitude squared, the same ratio yields twice as many decibels for amplitude as for power (coefficient 10 versus 20).
Use the same unit for input and output. Only the ratio matters, so any consistent unit such as watts or volts works.
Both the input and output value must be positive. The logarithm is undefined for values of zero or below.

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