Decibel Calculator

Convert between a ratio and decibels, for power or voltage/amplitude, and back. Includes common reference points.

Decibels
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Decibels are a logarithmic ratio. For power, dB = 10·log10(P2/P1); for voltage, current or amplitude, dB = 20·log10(V2/V1). So doubling power is +3dB, while doubling voltage is +6dB. 0dB means the two values are equal.

Frequently asked questions

Why are there two formulas, 10·log and 20·log?

Decibels measure power ratios with 10·log. Voltage, current and sound-pressure amplitude relate to power by a square, so converting them to dB uses 20·log. Using the wrong one gives an answer off by a factor of two in dB.

What do common dB values mean?

+3dB is double the power, +6dB is double the voltage or amplitude, +10dB is ten times the power, and +20dB is ten times the voltage. 0dB means no change (a ratio of 1).

Is this the same as dB for loudness?

This tool does relative ratios. Absolute units like dBSPL (loudness), dBm (power relative to 1mW) or dBV add a fixed reference; the underlying log maths is the same.