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Decibel (dB) Calculator

Compare two power, voltage or current values to get their ratio in decibels, or turn a level in dB back into a ratio. The answer is a relative level — how many times bigger one value is than the other — not an absolute power, voltage or current.

Formula

  • dB = 10 log10(P2 / P1)
  • dB = 20 log10(V2 / V1)
  • dB = 20 log10(I2 / I1)
  • P2 / P1 = 10^(dB / 10)
  • V2 / V1 = 10^(dB / 20)
  • I2 / I1 = 10^(dB / 20)

A decibel value describes a ratio, not an absolute level: it says how many times bigger one quantity is than another. Power uses 10 log10 because power is what the bel measures; voltage and current use 20 log10 because their square is proportional to power. That equivalence holds while both quantities are measured across the same impedance — across different impedances the same 20 log10 answer describes a voltage ratio, not a power ratio. Absolute references such as dBm, dBW or dBV are a different calculation and aren't covered here.

About this calculation

A decibel is a logarithmic way of writing a ratio. For power dB = 10 log10(P2 / P1); for voltage or current, measured in the same impedance, dB = 20 log10(V2 / V1). The logarithm compresses a huge range into a readable one: +3 dB is about double the power, +6 dB about double the voltage, and −20 dB a tenth of the voltage.

Assumptions and limits

  • The voltage and current forms assume both quantities are measured in the same impedance. Across different impedances the power form must be used instead.
  • The calculator works in ratios. Absolute levels such as dBm or dBu also need a reference level, and dB SPL needs the 20 µPa reference.
  • Results are rounded for display, so a value that should be exactly 0 dB may show as a very small number instead of zero.

A worked example

Doubling the power is 10 log10(2) ≈ +3.01 dB, while doubling the voltage is 20 log10(2) ≈ +6.02 dB — the two differ because power rises with the square of voltage.

See also Power and Nyquist Sampling.