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Fixed attenuators can be designed to have either equal or unequal impedances and to provide any amount of attenuation (theoretically) equal to or greater than the configuration's minimum attenuation  depending on the ratio of Z_{1}/Z_{2}. Attenuators with equal terminations have a minimum attenuation of 0 dB. Unequal terminations place a lower limit on the attenuation.
k = power in / power out:
, or in decibels
An online attenuator calculator is provided below.
Unbalanced Tee (T) Attenuator 
These equations apply to the two forms of Tee attenuators at the left.
If Z_{1} = Z_{2}, then:

Balanced Tee (T) Attenuator  
Unbalanced Pi (π) Attenuator 
These equations apply to the two forms of Pi attenuators at the left.
If Z_{1} = Z_{2}, then:

Balanced Pi (π) Attenuator  
Note: Only enter values in the yellow cells or risk overwriting formulas! 
An RF Cafe visitor wrote to say that he thought the above equations might be in error when unequal source and load termination resistances are used. The image below shows the mathematical steps that prove the equations are correct. It uses a source resistance of 50 ohms and a load resistance of 100 ohms, with an attenuation of 10 dB. Resistor values for both the "T" and ""Pi" attenuators were determined using the attenuator calculator on RF Cafe (which uses these equations).