Decibels for voltage and power
Why the decibel is a ratio on a log scale, when to use the factor of 20 or 10, and how to read common gain figures at a glance.
The decibel is a ratio, not a unit
A decibel never describes an absolute quantity on its own. It expresses how much larger or smaller one value is than a reference, placed on a logarithmic scale so that huge spans compress into readable numbers. That is why an amplifier is described as adding a certain gain in dB rather than a fixed number of volts. Because it is a pure ratio, the same decibel figure applies whether your reference is one volt, one milliwatt or last week's reading.
When to use 20 and when to use 10
Use the factor of 20 for amplitude quantities such as voltage, current or sound pressure, and the factor of 10 for power quantities such as watts. The split exists because power rises with the square of amplitude, so squaring inside the logarithm doubles the multiplier. This calculator asks you to pick voltage or power precisely so it applies the right factor, and mixing them up is the most common source of a result that is off by a factor of two.
Gain figures worth memorising
A few anchors make decibels intuitive. For voltage, a ratio of 2 is about 6 dB, a ratio of 10 is exactly 20 dB, and a ratio of 100 is 40 dB, so every extra factor of ten adds 20 dB. For power, doubling is close to 3 dB and a factor of ten is exactly 10 dB. Negative decibels simply mirror these, so minus 6 dB of voltage halves the signal. Keeping these landmarks in mind lets you sanity-check any number the calculator returns.
Why engineers reach for decibels
Chained stages make the case for decibels clear. When signals pass through several amplifiers and attenuators, their linear ratios multiply, which is fiddly to track. In decibels those same stages simply add and subtract, so a plus 20 dB preamp feeding a minus 6 dB pad gives a net plus 14 dB without a calculator. The log scale also matches human hearing and the vast dynamic range of real systems, which is why audio, radio and control engineering all speak in dB.