Decibel Calculator
Enter two values to find the ratio between them expressed in decibels, for both power and voltage/amplitude.
How decibels are calculated
The decibel (dB) is a logarithmic unit used to express a ratio between two values. For power quantities, dB = 10 × log₁₀(P2/P1). For voltage, current, or other amplitude quantities (which relate to power by a square law), dB = 20 × log₁₀(V2/V1) — the factor of 20 instead of 10 accounts for that squared relationship.
Because decibels are logarithmic, a 3 dB increase roughly doubles the power, while a 10 dB increase represents a tenfold increase in power. This compresses very large ranges of values — like sound intensity or signal strength — into more manageable numbers.
Frequently asked questions
Why are there two different formulas for decibels?
Power is proportional to the square of voltage or amplitude, so converting a voltage ratio to decibels uses a factor of 20 instead of 10 to keep the result consistent with the equivalent power ratio in dB.
What does a negative dB value mean?
A negative result means the measured value (P2) is smaller than the reference value (P1) — representing a reduction, loss, or attenuation rather than a gain.
Why use decibels instead of a plain ratio?
Decibels compress enormous ranges — like the ratio between the quietest and loudest sounds humans can hear — into much smaller, more intuitive numbers, and they let you add gains and losses together instead of multiplying ratios.