Attenuator calculator
Three resistors that have to survive
An attenuator is three resistors, and half the work is not calculating their values but not burning them out. The power divides between them very unevenly, and the first one in the signal path takes the larger share.
| Attenuation (dB) | |
| System impedance Z₀ (Ω) | |
| Circuit | |
| Input power (W) | |
| Second impedance (Ω), if the transition is asymmetric | |
| Values | |
| Shared | |
| Voltage ratio K | |
| Voltage at the input / output | |
| Output power | |
📐 The formulas
K = 10^(A/20) is the voltage ratio.
Pi arrangement:
R_shunt = Z₀ × (K + 1)/(K − 1)
R_series = Z₀ × (K² − 1)/(2K)
T arrangement:
R_series = Z₀ × (K − 1)/(K + 1)
R_shunt = 2 × Z₀ × K/(K² − 1)
Bridged-T (the arms equal Z₀):
R1 = Z₀ × (K − 1) R2 = Z₀ / (K − 1) R1 · R2 = Z₀²
L-pad (matched on one side):
R_series = Z₀ × (K − 1)/K R_shunt = Z₀ / (K − 1)
Minimum attenuation of an asymmetric transition:
A_min[dB] = 20 × log10( √(Z_hi/Z_lo) + √(Z_hi/Z_lo − 1) )
Check values for 50 Ω:
| Attenuation | Pi: shunt / series | T: series / shunt |
|---|---|---|
| 3 dB | 292.4 / 17.6 Ω | 8.55 / 141.9 Ω |
| 10 dB | 96.2 / 71.2 Ω | 25.97 / 35.14 Ω |
| 20 dB | 61.1 / 247.5 Ω | 40.9 / 10.1 Ω |
Where this calculator stops:
- The power split assumes a matched attenuator: with a different load it is different
- The formulas hold for a symmetric Z₀ → Z₀ transition; for an asymmetric one the calculator only gives the minimum attenuation
- Lead inductance and package capacitance are not accounted for: above 1 GHz they spoil the match
- An L-pad is matched on one side only — on the other it presents a mismatch
🎯 In practice
- Protecting a NanoVNA input from too much power.
- Isolating stages so that one does not see the other's mismatch.
- Reducing a generator's output without changing the match.
Practical advice:
- The input resistor gets hottest: the shunt one in a Pi, the series one in a T.
- Bridged-T suits variable attenuators: only two resistors change, the arms stay put.
- At RF use leadless resistors; at 1 GHz even an 0805 has noticeable inductance.
- A 6 dB pad at an instrument's input turns any load SWR into one no worse than 1.2.
© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.
How to cite this calculator
UR3PKI. «Attenuator calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/attenuator (licence CC BY-NC-SA 4.0).The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.