Pi & T Attenuator Calculator
Resistor values from attenuation and impedance — or enter the resistors you have to get the attenuation and matched impedance.
Edit the left column to size a pad — or type into R1·R2·R3 to analyse resistors you already have. Every box is live.
Resistive pads: fixed loss, matched impedance
An attenuator trades signal level for a controlled, matched impedance at both ports. Express the attenuation as a voltage ratio
For a symmetric pad of impedance the resistor values are closed-form. The T-pad (two series arms, one shunt):
The Pi-pad (one series arm, two shunts) is its dual:
The reverse problem — resistors in, attenuation and impedance out — comes from the network's image parameters. From the ABCD matrix of the pad the matched port impedances and attenuation are
When the pad also matches between the two impedances, but only above a minimum attenuation — below it the resistors would be negative. The tool solves the general asymmetric case and reports that minimum loss for your impedances.
Frequently asked questions
What is the difference between a Pi and a T attenuator?
Both are resistive pads that drop the signal by a fixed amount while keeping the line impedance matched. A T-pad uses two series resistors with one shunt to ground in the middle; a Pi-pad uses one series resistor with a shunt to ground at each end. They are electrically equivalent in attenuation and matching — the choice is practical: T-pads tend to give more convenient resistor values at high attenuation, Pi-pads at low attenuation, and board layout or parasitics may favour one.
How do I calculate attenuator resistor values?
From the attenuation A in dB, form the voltage ratio K = 10^(A/20). For a matched pad of impedance Z0: T-pad → R1 = R2 = Z0·(K−1)/(K+1) and R3 = Z0·2K/(K²−1); Pi-pad → R1 = R2 = Z0·(K+1)/(K−1) and R3 = Z0·(K²−1)/(2K). Example: 3 dB into 50 Ω gives a T-pad of 8.55 Ω, 8.55 Ω and 141.9 Ω.
Can I work backwards from resistors I already have?
Yes. Enter values in the R1, R2, R3 boxes and the tool solves the reverse problem from the network's image parameters: the attenuation those resistors produce and the input/output impedances at which the pad is matched. This is handy when you want to build a pad from the standard resistors in your drawer rather than buy exact values.
Can the input and output impedances be different?
Yes — the tool handles the general asymmetric pad, so Zin and Zout can differ. But unequal impedances impose a minimum attenuation: you cannot match 75 Ω to 50 Ω in a single pad with less than about 5.7 dB of loss. Below that minimum the resistor values would go negative, so the tool reports the minimum attenuation your impedances allow.