T Attenuator Calculator

This T attenuator calculator works out the resistor values you need to build a symmetric T-pad, a three-resistor network wired in a T shape that drops a signal by a set amount in decibels while keeping the source and load impedances matched. You enter two figures: the attenuation you want in dB and the system impedance Z0, typically 50 ohms for radio frequency work or 75 ohms for video. From these it returns the value of each of the two equal series resistors (R1), the single shunt resistor (R2), and the voltage ratio K, ten raised to the power of the decibels divided by twenty, which drives both resistor formulas. T-pads are a standard way electronics designers, hobbyists and students pad down a signal without introducing reflections, and they do the same job as a Pi-pad using a different resistor arrangement. Type in your target attenuation and impedance and the three results update instantly, so you can try different dB steps or impedances before you order or hand-select resistors. The formula, a worked example at 6 dB and 50 ohms, and the assumptions behind the calculation, including that it treats components as ideal and rounds results for display, are set out below. Because real resistors come in fixed preferred values, treat the figures as a design target to match with the nearest standard part rather than an exact specification.

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Attenuation (dB)

Impedance Z0 (Ω)

Shunt resistor R2

Ratio K = 10^(dB/20)

The formula

Let K = 10 to the power (dB / 20). For a symmetric T-pad of impedance Z0, each series resistor is R1 = Z0 (K minus 1) / (K + 1), and the shunt resistor is R2 = Z0 times 2K / (K squared minus 1).

Worked example

For 6 dB at 50 ohms, K = 1.995, so each series resistor is about 16.6 ohms and the shunt resistor about 66.9 ohms. Enter 6 and 50 to confirm.

Frequently asked questions

What is a T attenuator?

A three-resistor network (two series, one shunt) shaped like a T that attenuates a signal while matching impedance.

How does it differ from a Pi-pad?

It uses two series and one shunt resistor rather than one series and two shunt, but achieves the same attenuation and matching.

What is the impedance Z0?

The system impedance the pad is designed to match, commonly 50 ohms for radio or 75 ohms for video.

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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