Decibel Calculator

The bel — named for Alexander Graham Bell — was created at Bell Telephone Laboratories in 1924 by Colonel William H. Martin to quantify signal losses on long telephone cables. One bel represents a tenfold change in power; the bel was quickly divided into ten "decibels" for practical everyday use, giving the familiar ~1 dB just-noticeable-difference in human hearing.

The logarithmic scale matches the natural psychophysics of human perception. Gustav Fechner\'s 1860 Weber-Fechner law established that perceived intensity of sound, light, and pressure is proportional to the logarithm of the physical stimulus — so plotting loudness, brightness, or pressure on a log scale gives approximately linear perceived changes. This is why decibels and musical octaves (which are log₂) both work so well for describing sensory phenomena.

The distinction between 10·log (for power) and 20·log (for voltage or current) comes from the fact that power = V²/R, so a factor-of-2 voltage change corresponds to a factor-of-4 power change. Both express the same physical change — 10·log(4) = 20·log(2) ≈ 6.02 dB — so the two conventions give identical results for identical physical situations. Engineers use whichever matches their measurement (voltmeter → 20·log; wattmeter → 10·log).

Pick conversion direction (ratio → dB or dB → ratio) and measurement type (power 10·log vs voltage 20·log). The tool converts between linear ratio and decibels, also showing the equivalent in the other unit type (e.g., 20 dB voltage = 100× voltage = 10,000× power). Use quick presets for common benchmarks (2×, 10×, 100×).

Reference tables below show common absolute-reference decibel scales: dBm (mW), dBW (W), dBV (1 V RMS), dBu (0.775 V RMS), dBSPL (20 µPa for acoustic sound), dBi (isotropic radiator for antennas). When you see "dBm," "dBV," etc., that\'s an absolute reference — plain "dB" is just a ratio. Everything runs client-side; no values leave your browser.