When do I use 10·log versus 20·log?

Use 10·log₁₀(P/P0) for power ratios. Use 20·log₁₀(A/A0) for amplitude quantities like voltage, current or sound pressure, because power is proportional to amplitude squared.

What does a negative dB value mean?

A negative result means the measured value is smaller than the reference — an attenuation or loss rather than a gain.

How much louder is +3 dB and +10 dB?

On a power scale, +3 dB is roughly double the power (10·log₁₀(2) ≈ 3.01 dB) and +10 dB is exactly ten times the power. For amplitude quantities, +6 dB is double the amplitude (20·log₁₀(2) ≈ 6.02 dB).

Does this calculate sound pressure level (SPL)?

It calculates the dB ratio between any two values. For SPL, choose amplitude mode and use the reference sound pressure (20 micropascals) as the reference value; the result is the level in dB SPL.

Why are decibels logarithmic?

Human senses like hearing respond roughly logarithmically, and the quantities involved span many orders of magnitude. A log scale compresses that huge range into manageable numbers where equal dB steps mean equal ratios.

0 dB means the measured value equals the reference value, because log₁₀(1) = 0. It indicates no gain and no loss relative to the reference, not the absence of signal.

Decibel (dB) Calculator

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No — every calculation runs locally in your browser, with no network requests.

Decibel (dB) calculator

A decibel expresses a ratio between two values on a logarithmic scale. This calculator converts a power or amplitude ratio into decibels — useful in audio, electronics, RF and acoustics, where gains, losses and signal levels are quoted in dB.

How it works

The formula depends on the type of quantity:

Power (watts): dB = 10 · log₁₀(P / P₀)
Amplitude (voltage, current, sound pressure): dB = 20 · log₁₀(A / A₀)

The amplitude form uses 20 instead of 10 because power is proportional to amplitude squared, and log of a square doubles the coefficient. Both values must be greater than zero. A positive result is a gain; a negative result is a loss (attenuation); zero means the values are equal.

Example

Measured power of 2 W against a reference of 1 W, in power mode:

dB = 10 · log₁₀(2 / 1) = 10 · 0.3010 = 3.01 dB

The same 2:1 ratio in amplitude mode (e.g. 2 V vs 1 V) gives 20 · log₁₀(2) = 6.02 dB.

Ratio	Power (10·log)	Amplitude (20·log)
2 : 1	+3.01 dB	+6.02 dB
10 : 1	+10 dB	+20 dB
1 : 2	−3.01 dB	−6.02 dB
1 : 1	0 dB	0 dB

Every calculation runs locally in your browser, with no network requests.