Calculate T-pad and Pi-pad RF attenuator resistor values for any attenuation (dB) and reference impedance (Z₀). Formulas, interactive calculator, and standard attenuation table.
Interactive Attenuator Calculator
T-Pad and Pi-Pad Attenuator Design
T-Pad (balanced)
R1 = R3 (series arms)
—
R2 (shunt arm)
—
Pi-Pad
R1 = R3 (shunt arms)
—
R2 (series arm)
—
Performance
S11 = S22
—
Power dissipated (for 0 dBm in)
—
Attenuator Formulas
Let K = 10^(A_dB / 20)
T-Pad (balanced, 3 resistors):
R1 = R3 = Z₀ · (K − 1) / (K + 1) [series arms, both equal]
R2 = Z₀ · 2K / (K² − 1) [shunt arm between the two series resistors]
Pi-Pad (3 resistors):
R1 = R3 = Z₀ · (K + 1) / (K − 1) [shunt arms to ground, both equal]
R2 = Z₀ · (K² − 1) / (2K) [series arm between shunt resistors]
For a matched attenuator: S11 = S22 = −∞ dB (perfectly matched to Z₀)
S21 = S12 = −A dB (loss = attenuation setting)
Standard Attenuator Values (Z₀ = 50 Ω)
A (dB)
K
T-pad R1=R3 (Ω)
T-pad R2 (Ω)
Pi-pad R1=R3 (Ω)
Pi-pad R2 (Ω)
1
1.122
2.88
433.3
869.5
5.77
2
1.259
5.73
215.2
436.2
11.61
3
1.413
8.55
141.9
292.4
17.61
6
1.995
16.61
66.93
150.5
37.35
10
3.162
25.97
35.14
96.25
71.15
20
10.00
40.91
10.10
61.11
247.5
30
31.62
48.44
3.17
51.67
790.3
40
100.0
49.50
1.01
50.50
2500
Practical Design Notes
Use 1% tolerance resistors for attenuation accuracy better than 0.5 dB
For >10 GHz: use thin-film resistors on RF-grade substrate (not wirewound)
Power rating: each resistor must handle its share of dissipated power
For high-power attenuators (>1W): use 0805 or 1206 SMD, or specialized thick-film pads
T-pad is preferred for low attenuation (1–6 dB); Pi-pad for high attenuation (10–40 dB)
RF View: The RF View Utilities tab includes the Attenuator Calculator with T-pad and Pi-pad output. The result can be loaded directly into the Circuit Simulator as a 2-port attenuator block for verifying S11 and S21 in your matching chain.