Pi Attenuator Calculator
This Pi attenuator calculator gives the resistor values for a symmetric Pi-pad, a three-resistor network that reduces a signal by a set amount while keeping the source and load impedance matched. Enter the attenuation in decibels and the system impedance, and the calculator returns the single series resistor and the two equal shunt resistors. Pi-pads are widely used in radio and test equipment to set precise signal levels without reflections. The values come from the voltage ratio K, which is ten to the power of the decibels over twenty. The formula, a worked example and the assumptions are below.
The formula
Let K = 10 to the power (dB / 20). For a symmetric Pi-pad of impedance Z0, the series resistor is R2 = Z0 (K squared minus 1) / (2K), and the two shunt resistors are each R1 = Z0 (K + 1) / (K minus 1).
Worked example
For 6 dB at 50 ohms, K = 10 to the 0.3 = 1.995, so the series resistor is about 37.3 ohms and each shunt resistor about 150.5 ohms. Enter 6 and 50 to confirm.
Frequently asked questions
What is a Pi attenuator?
A three-resistor network (one series, two shunt) shaped like the Greek letter Pi that attenuates a signal while matching impedance.
What does K mean?
The voltage ratio of the attenuation, K = 10 to the power of the decibels divided by 20.
When use Pi instead of T?
Both achieve the same attenuation and matching; the choice often comes down to which resistor values are more convenient.
Who this calculator is for
This calculator is for electronics students, hobbyists and engineers.
What this calculator assumes
- You enter values in the units shown.
- Ideal components are assumed.
- Results are rounded for display.
Formula and sources
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