Pi Attenuator Calculator
Design resistive Pi and T attenuator pads. Enter the attenuation in dB and the system impedance in ohms to get the exact resistor values, the nearest E12/E24 standard parts, the real built attenuation, and per-resistor power dissipation. Everything runs locally in your browser.
Attenuator designer
Results
Schematic
Resistor values
| Resistor | Ideal | Nearest E24 | Error |
|---|
Built performance (with nearest E24 parts)
Power dissipation
| Resistor | Dissipation |
|---|
Rule of thumb: use parts rated for at least 2× the calculated dissipation.
Worked examples
Example 1 — 3 dB Pi pad for 50 Ω
- Voltage ratio: K = 103/20 = 100.15 = 1.41254.
- Series resistor: Rseries = 50 × (1.41254² − 1) / (2 × 1.41254) = 50 × 0.99526 / 2.82508 = 17.61 Ω.
- Shunt resistors (two of them): Rshunt = 50 × (1.41254 + 1) / (1.41254 − 1) = 50 × 2.41254 / 0.41254 = 292.40 Ω.
- Nearest E24 parts: 18 Ω (+2.19%) and 300 Ω (+2.60%). Built attenuation: 2.9946 dB — only 0.0054 dB off target.
Example 2 — 10 dB Pi pad for 75 Ω (video)
- K = 1010/20 = 3.16228.
- Rseries = 75 × (10 − 1) / (2 × 3.16228) = 106.73 Ω.
- Rshunt = 75 × 4.16228 / 2.16228 = 144.37 Ω (two of them).
- Nearest E24: 110 Ω (+3.07%), 150 Ω (+3.90%) → actual 9.9662 dB.
Example 3 — 6 dB T pad for 50 Ω
- K = 106/20 = 1.99526.
- Series arms (two): Rarm = 50 × (1.99526 − 1) / (1.99526 + 1) = 50 × 0.99526 / 2.99526 = 16.61 Ω each.
- Center shunt: Rshunt = 50 × 2 × 1.99526 / (1.99526² − 1) = 50 × 3.99052 / 2.98107 = 66.93 Ω.
- Nearest E24: 16 Ω (−3.70%), 68 Ω (+1.60%) → actual 5.8477 dB.
Design formulas
With attenuation A in dB and system impedance Z₀, first compute the voltage ratio:
Pi network — one series resistor, two shunt resistors:
T network — two series arms, one center shunt:
Both topologies are symmetric and present Z₀ at input and output when built with the ideal values.
Frequently asked questions
What is a Pi attenuator?
A Pi attenuator is a resistive pad whose schematic looks like the Greek letter π: two shunt resistors to ground with one series resistor between them. It reduces signal level by a fixed amount in dB while keeping the input and output matched to the system impedance, typically 50 or 75 ohms.
What formulas does this Pi attenuator calculator use?
First the voltage ratio K = 10^(A/20), where A is the attenuation in dB. For the Pi network: Rseries = Z0 × (K² − 1) / (2 × K) and Rshunt = Z0 × (K + 1) / (K − 1). For the T network: each series arm Rarm = Z0 × (K − 1) / (K + 1) and the center shunt Rshunt = Z0 × 2K / (K² − 1).
Should I choose a Pi or a T attenuator?
Electrically they are equivalent when both are matched to the same impedance, so pick whichever gives resistor values closer to standard E12 or E24 parts. Pi pads are the most common choice in RF work at 50 ohms, while T pads are handy when the Pi shunt values become impractically large or small.
Why do the calculated resistor values not match E12 or E24 parts?
The ideal values are exact mathematical solutions and rarely land on a standard value. Use the built-in E12/E24 finder to snap each resistor to the nearest standard part. The calculator shows the percentage error and recomputes the actual attenuation of the built pad so you can see the effect before soldering.
What is the K factor in attenuator design?
K is the linear voltage ratio corresponding to the attenuation: K = 10^(A/20). A 3 dB pad has K = 1.4125, a 6 dB pad has K = 1.9953, a 10 dB pad has K = 3.1623, and a 20 dB pad has K = 10. All resistor formulas are written in terms of K and the system impedance Z0.
What power rating do the resistors need?
Enter your input power and the calculator shows how many watts each resistor dissipates. As a rule of thumb, choose resistors rated for at least twice the calculated dissipation. The input shunt resistor in a Pi pad and the first series arm in a T pad usually run hottest.
Does the attenuation stay flat with frequency?
Ideal resistors give the same attenuation at every frequency. In a real build, lead inductance and stray capacitance degrade performance at VHF and above, so keep leads short, use a compact layout, and prefer surface-mount or metal-film parts for RF pads.
What happens with very small or very large attenuation values?
As the attenuation approaches 0 dB, the series resistor approaches 0 ohms and the shunt resistors approach infinity, so pads below about 1 dB are impractical. As attenuation grows large, the series resistor grows without bound (toward an open circuit) while the shunt resistors approach Z0; values above 40 dB are usually built as two cascaded pads.
Can I use a Pi attenuator for audio or video?
Yes, as long as you design for the right impedance: 600 ohms for pro audio lines, 75 ohms for video. The same formulas apply at any impedance. For unbalanced-to-balanced or bridging applications, check that the pad does not disturb the source and load matching your system expects.
Is this calculator free and private?
Yes. It runs entirely in your browser with no account, no cookies, and no network requests. Your attenuation, impedance, and power values never leave your device.
Free tool by NRTechStudio