Calculate output voltage, input voltage, or attenuation in dB from any two values, with mV, V, and kV unit options for voltage attenuation.
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Attenuation Formula
The attenuation calculator uses the voltage ratio form of the decibel formula. Enter any two values: output voltage, input voltage, or attenuation in dB. The missing value is calculated from the same relationship.
A = 20*log_10(V_in/V_out)
V_out = V_in/10^(A/20)
V_in = V_out*10^(A/20)
- A = attenuation in decibels, dB
- V_in = input voltage
- V_out = output voltage after attenuation
- log_10 = base-10 logarithm
The calculator converts voltage entries to volts before applying the formula. Millivolts, volts, and kilovolts can be used for the input and output voltage fields.
- Find attenuation: use input voltage and output voltage to calculate the dB loss.
- Find output voltage: use input voltage and attenuation to calculate the reduced voltage level.
- Find input voltage: use output voltage and attenuation to calculate the voltage before attenuation.
Common Voltage Attenuation Ratios
These values show how common attenuation levels compare to voltage ratios.
| Attenuation | Voltage Ratio Vout / Vin | Meaning |
|---|---|---|
| 0 dB | 1.000 | No attenuation |
| 3 dB | 0.708 | Output voltage is about 70.8% of input |
| 6 dB | 0.501 | Output voltage is about half of input |
| 10 dB | 0.316 | Output voltage is about 31.6% of input |
| 20 dB | 0.100 | Output voltage is one-tenth of input |
| 40 dB | 0.010 | Output voltage is one-hundredth of input |
| Unit | Equivalent in Volts | Example |
|---|---|---|
| mV | 0.001 V | 500 mV = 0.5 V |
| V | 1 V | 2 V = 2 V |
| kV | 1000 V | 1.5 kV = 1500 V |
Example Problems
Example 1: Calculate attenuation
You have an input voltage of 10 V and an output voltage of 1 V.
A = 20*log_10(10/1)
A = 20 dB
The attenuation is 20 dB.
Example 2: Calculate output voltage
You have an input voltage of 5 V and attenuation of 6 dB.
V_out = 5/10^(6/20)
V_out = 2.5059 V
The output voltage is about 2.506 V.
FAQ
Why does the formula use 20 instead of 10?
Voltage attenuation uses 20 log because voltage is a field quantity. Power ratios use 10 log. If you are comparing voltages across the same impedance, the voltage dB formula is:
A = 20*log_10(V_in/V_out)
Can attenuation be negative?
Yes. If the output voltage is greater than the input voltage, the formula gives a negative attenuation value. That means the signal has gained voltage instead of being reduced. For example, if 1 V goes in and 2 V comes out, the result is about -6.02 dB.
Do input and output voltage need to use the same unit?
No. You can enter one voltage in mV and the other in V or kV. The calculator converts both values to volts before calculating attenuation, input voltage, or output voltage.
