dB (Decibel) Calculator

What Is a Decibel?

The decibel (dB) is a logarithmic unit used to express the ratio of two quantities — most commonly power or amplitude (voltage/current). Because it is logarithmic, dB values can be added and subtracted where ratios would need to be multiplied and divided, making them extremely convenient for signal chain analysis.

A positive dB value indicates gain (the output is larger than the input). A negative value indicates loss (attenuation). Zero dB means unity gain — no change.

Power vs Voltage dB

The formula differs depending on whether you are measuring power or amplitude (voltage or current):

• Power: dB = 10 × log₁₀(Pâ‚‚ / P₁)
• Voltage / Amplitude: dB = 20 × log₁₀(Vâ‚‚ / V₁)

The factor of 20 for voltage arises because power is proportional to the square of voltage (P ∝ V²), so 10 × log₁₀(V²) = 20 × log₁₀(V).

Common dB Reference Points

• +3 dB (power): power doubles (ratio ≈ 2)
• −3 dB (power): power halves — the −3 dB point defines filter cutoff frequency
• +6 dB (voltage): voltage doubles (ratio = 2)
• +20 dB (voltage): voltage × 10 (ratio = 10)
• 0 dB: ratio = 1, unity gain