Decibel Voltage Ratio Calculator

How it is calculated

The tool computes dB = 20 × log₁₀(Vout / Vin). Vout and Vin are voltages in the same unit (volts, millivolts, RMS or peak, as long as both match). Only their ratio matters. The result is in decibels, rounded to two decimals; positive means gain, negative means attenuation, and 0 dB means equal voltages.

Both voltages must be greater than zero because the logarithm of zero or a negative number is undefined; otherwise the tool returns nothing. The factor 20 is for voltage (a field quantity). It assumes both voltages are measured across the same impedance, which is the case when you compare a gain stage or filter. For power ratios the factor would be 10.

Common mistakes

• Using the power formula (10 × log) on voltages. That gives half the correct dB; voltage ratios use 20.
• Mixing units, such as Vout in millivolts and Vin in volts. The ratio then comes out 1,000 times off, adding 60 dB.
• Swapping input and output. The sign flips, so a gain of +13.98 dB would show as −13.98 dB.
• Mixing RMS with peak values. Both voltages should be measured the same way.
• Entering a negative or zero voltage to represent a phase flip or no signal. The tool needs positive magnitudes and gives no result otherwise.