Pi Attenuator Calculator

Calculate matched Pi attenuator resistor values, voltage ratio, and insertion loss for RF and audio impedance systems.

Design a symmetrical three-resistor Pi pad for equal source and load impedances.

System Impedance (Ω)

Attenuation (dB)

About Pi attenuator design

A Pi attenuator is a passive three-resistor network shaped like the Greek letter pi. Two shunt resistors connect the input and output nodes to ground, while one series resistor joins those nodes. The network reduces signal voltage and power while presenting the desired impedance at both ports, making it useful between matched RF stages, measurement equipment, audio circuits, and transmission lines. This calculator designs a symmetrical pad whose source and load impedances are equal. It converts attenuation in decibels to the voltage ratio factor K using K = 10 raised to attenuation divided by 20. Each shunt resistor is Z times (K + 1) divided by (K - 1). The series resistor is Z times (K squared - 1) divided by 2K. The displayed voltage ratio is 1 divided by K. Matching matters because an arbitrary resistor divider changes the impedance seen by both source and load. Reflections can distort RF measurements and reduce delivered power when a line is not terminated in its characteristic impedance. A correctly calculated pad maintains nominal impedance when both ends are properly terminated. Common systems use 50 Ω for RF instruments, 75 Ω for video and cable applications, or 600 Ω in legacy audio interfaces. Real resistors have tolerances and parasitic inductance and capacitance. Choose available values near the calculated targets, then evaluate the resulting attenuation and return loss. Thin-film resistors with tight tolerance are generally preferable for precision. At high frequencies, package size and layout become part of the network; short connections, a continuous ground, and symmetric placement reduce unwanted reactance. All dissipated signal power becomes heat in the resistors. Check each component's power rating, especially for low attenuation or high input power. This ideal calculator assumes purely resistive equal impedances and does not model frequency response, unequal terminations, reactive loads, or maximum power. Verify a practical design with circuit simulation or a network analyzer across the intended bandwidth.

Pi attenuator examples

50 Ω, 6 dB	Shunt 150.48 Ω; series 37.35 Ω	Common RF level reduction.
75 Ω, 10 dB	Shunt 144.37 Ω; series 106.73 Ω	Matched video-system pad.
50 Ω, 20 dB	Shunt 61.11 Ω; series 247.50 Ω	Strong attenuation requires a large series value.

How to calculate a Pi attenuator

Enter the equal source and load system impedance.
Enter the required positive attenuation in decibels.
Click Calculate Resistors and note both identical shunt values and the series value.
Select practical precision resistors and verify loss and power rating.

Pi attenuator FAQ

Are the two shunt resistors equal?

Yes, this calculator designs a symmetrical pad for equal source and load impedances. Unequal impedances require different equations and usually unequal shunt values.

Can I use standard resistor values?

Choose the nearest available precision values when exact values are unavailable. Simulate or measure the resulting attenuation and return loss because rounding changes both.

Why must attenuation be above zero?

At zero decibels the ideal shunt resistors become infinite and the series resistor becomes zero. That condition is simply a direct connection rather than an attenuator.

Does the pad handle high power?

The equations determine resistance but not component wattage. Calculate resistor dissipation from input power and apply appropriate thermal derating.