Pi Attenuator Calculator (π-pad resistor values from dB and impedance, plus T-pad)

Understanding the pi attenuator calculator — concept and principle

This calculator gives the resistor values for a π (pi) attenuator pad that reduces a signal by a chosen number of decibels while staying matched to the system impedance, and also the equivalent T-pad values. RF technicians, audio engineers and test-lab staff use it to build fixed attenuators.

Two shunts and one series resistor cut the power tenfold while both ends still look like 50 Ω.

How it is calculated

K = 10^(dB/20) (voltage ratio)

π pad: R_shunt = Z₀ (K + 1) ÷ (K − 1); R_series = Z₀ (K² − 1) ÷ (2K)

T pad: R_series = Z₀ (K − 1) ÷ (K + 1); R_shunt = 2K Z₀ ÷ (K² − 1)

Power ratio = K²; output power share = 1 ÷ K²

The principle

An attenuator must do two things: reduce the signal by the right amount and look like the system impedance from both ends, so it causes no reflections. A π pad has two shunt resistors to ground with a series resistor between them. Solving the two conditions, correct loss and matched input and output, gives the formulas. The T pad does the same job with the resistors arranged the other way round.

Worked example

Default inputs: 10 dB, Z₀ = 50 Ω.

K = 10^(10/20) = 3.1623; power ratio = 10; output share = 10%.
R_shunt = 50 × 4.1623 ÷ 2.1623 = 96.25 Ω each.
R_series = 50 × (10 − 1) ÷ 6.3246 = 71.15 Ω.
T pad: series 25.97 Ω each, shunt 35.14 Ω.

Each point is the calculator re-run with only “Attenuation (dB)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Attenuation (dB)	Shunt resistors R1, each (Ω)
2.5	349.83
5	178.49
7.5	122.92
10	96.25
12.5	81.09
15	71.63
20	61.11

How to read the result

Round to the nearest standard resistor values; small errors slightly change both the attenuation and the match. At high frequencies use non-inductive thin-film or SMD resistors with short leads. Higher attenuation means smaller shunt and larger series resistors in a π pad.

Common mistakes

Using 10 log instead of 20 log to get the voltage ratio.
Building a pad for 50 Ω and using it in a 75 Ω system.
Ignoring the power rating of the first shunt and series resistors.

Keep in mind: Check resistor power ratings and frequency behaviour. At RF, layout and parasitics affect performance.

Frequently asked questions

What is the difference between π and T pads?

π uses two shunt and one series resistor; T uses two series and one shunt. Both can match the same impedance.

Does the pad work in both directions?

Yes, symmetric pads are bidirectional.

How much power reaches the output at 10 dB?

One tenth.

Can I stack pads?

Yes; their dB values add.

Sources: Pulse Research Lab, application note "Build Your Own Custom Attenuators (Broadband and Precision)" (2012)

Checked 9 Nov 2026. Pulse Research Lab, Rapid Application Note “Build Your Own Custom Attenuators (Broadband and Precision)”, Rev. 4.2.2012: Eqs. (3)–(6) for Pi and Tee attenuators with α = V_IN/V_O.

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