Pi Attenuator Calculator — RF & Audio Pad Design
Design a symmetric Pi attenuator pad: enter the required attenuation in dB and the source/load impedance, and get the three resistor values — two shunt resistors and one series resistor — for a perfectly matched circuit.
dB
Ω
Two identical shunt resistors to ground at the input and output ports
- 1
Voltage amplitude ratio K
10^(6 ÷ 20) = 1.9953Ratio of input to output voltage amplitude; K > 1 means attenuation. - 2
Shunt resistors R1 = R3
50 × (1.9953 + 1) ÷ (1.9953 − 1) = 150.48
How does this calculator work?
For a symmetric Pi pad: K = 10^(A/20); shunt R1 = R3 = Z₀(K+1)/(K−1); series R2 = 2Z₀K/(K²−1). Example: 6 dB, 50 Ω → R1 = R3 ≈ 150 Ω, R2 ≈ 37 Ω. Preserves impedance matching at both ports across all frequencies.
Formula
How this is calculated
A Pi attenuator (named after the Greek letter π because its schematic shape resembles that letter) is a purely resistive network that reduces signal level while keeping both ports matched to the characteristic impedance. It has one series resistor (R2) in the signal path and one shunt resistor to ground on each side (R1 at input, R3 at output).
For a symmetric design — source and load both equal to Z₀ — the voltage amplitude ratio K equals 10^(A/20), where A is the attenuation in dB. The shunt resistors are Z₀(K+1)/(K−1) and the series resistor is 2Z₀K/(K²−1). These exact formulas ensure the input and output each look like Z₀, eliminating reflections regardless of frequency — the pad is wideband and flat to DC.
In practice you choose the nearest standard E12, E24 or E96 series resistor value. Very small attenuations (< 1 dB) produce very high shunt resistance and very low series resistance; high attenuations (> 30 dB) push the shunt resistors towards Z₀ and demand tight tolerances. Common impedances are 50 Ω (RF coaxial), 75 Ω (cable TV/video), 110 Ω (balanced digital audio), and 600 Ω (telephone/professional audio).
Frequently asked questions
Both achieve the same attenuation for the same impedance. A Pi pad has two shunt (parallel-to-ground) resistors and one series resistor; a T pad reverses this — two series resistors and one shunt. Both are equally valid; Pi pads are preferred at very high frequencies because the first shunt capacitance is in parallel with a relatively high impedance, which is less harmful to bandwidth than a series element with stray capacitance.
Yes — just enter your Z₀. The formulas scale exactly. Common values: 50 Ω (RF coax), 75 Ω (cable TV/video), 110 Ω (balanced AES/EBU), 600 Ω (telephone/studio). The calculated values assume the source and load both equal Z₀; if they differ, use an asymmetric L-pad or bridged-T design instead.
At exactly 0 dB (K = 1) the shunt resistors become infinite (open circuit) and the series resistor becomes 0 Ω — that is simply a wire with no pad. Negative dB would imply gain, which a passive resistive network cannot provide. Minimum useful attenuation is around 1 dB; below that, the series resistor is very small and sensitive to lead inductance.
Also known as
TG we-Calculate Editorial Team. (2026). Pi Attenuator Calculator — RF & Audio Pad Design [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/pi-attenuator-calculator
TG we-Calculate Editorial Team. "Pi Attenuator Calculator — RF & Audio Pad Design." TG we-Calculate. 2026. https://we-calculate.com/calculator/pi-attenuator-calculator.
TG we-Calculate Editorial Team, "Pi Attenuator Calculator — RF & Audio Pad Design," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/pi-attenuator-calculator
@misc{wecalculate_pi_attenuator_calculator, title = {Pi Attenuator Calculator — RF & Audio Pad Design}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/pi-attenuator-calculator}}, year = {2026}, note = {TG we-Calculate} }
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