🔊 Decibel (dB) Calculator
Convert a ratio to decibels or decibels to a ratio, for power (10·log) or amplitude/voltage (20·log).
Decibels
3.0103 dB
Decibels express a ratio on a log scale. For power, dB = 10·log₁₀(P₂/P₁), so ×2 power ≈ +3 dB and ×10 = +10 dB. For amplitude (voltage, current, sound pressure), dB = 20·log₁₀(A₂/A₁), so ×2 voltage ≈ +6 dB and ×10 = +20 dB. 0 dB means the two are equal. 🔒 In your browser.
How the decibel (db) calculator works
The decibel expresses a ratio between two quantities on a logarithmic scale. For power, dB = 10·log₁₀(P₂/P₁); for amplitude quantities like voltage, current or sound pressure, dB = 20·log₁₀(A₂/A₁) — the factor of 20 because power goes as amplitude squared. The tool converts either way: enter a ratio to get decibels, or a decibel value to get the ratio back (10^(dB/10) for power, 10^(dB/20) for amplitude). Pick power or amplitude and the direction.
Remember the landmark values: +3 dB is roughly double the power, +6 dB double the voltage, +10 dB ten times the power and +20 dB ten times the voltage. 0 dB means the two quantities are equal, and negative decibels mean a reduction. Decibels are dimensionless — a "dB" only has meaning relative to a stated reference (dBm, dBV, dB SPL name theirs).
Frequently asked questions
How do I convert a ratio to decibels?
For power, dB = 10·log₁₀(ratio); for voltage or other amplitudes, dB = 20·log₁₀(ratio). So ×2 power is about +3 dB, and ×2 voltage is about +6 dB. Enter your ratio and pick the type.
Why is it 10·log for power but 20·log for voltage?
Power is proportional to voltage (or amplitude) squared. Taking the log brings the exponent 2 out front, turning 10·log of the power ratio into 20·log of the voltage ratio — the same decibel result expressed in different quantities.
What does +3 dB mean?
About double the power (10·log₁₀2 ≈ 3.01 dB). For voltage, doubling is about +6 dB. −3 dB is roughly half the power, which is why the −3 dB point marks a filter's cutoff (half-power) frequency.
What is 0 dB?
A ratio of 1 — the two quantities are equal, since log₁₀(1) = 0. Positive decibels indicate a gain (increase), negative decibels a loss (decrease).
Are decibels absolute or relative?
Relative by themselves — a decibel is a ratio. Absolute decibel units name a reference: dBm is relative to 1 milliwatt, dBV to 1 volt, dB SPL to the threshold of hearing. This calculator works with the underlying ratios.