Calculating the prototypical filter requires knowing the order of the filter. It is possible to select an order based on past experience. There is a formula based on the corner frequency and the amount of attenuation desired at a selected frequency in the stop band. From time to time this version of the formula is given: Another variation has this denominator 
The formula for order determination is sometimes shown as The terms f_{c} and f_{s} are self explanatory in the case of low pass filters. Some allowances must be made for high pass filters so that f_{s} is the larger quantity, so the variables are renamed. In the case of band pass filters, f_{s} is the stop bandwidth and f_{c} is the pass bandwidth. This is done so that f_{s} is the larger quantity. For band stop (and notch) filters, f_{s} the stop bandwidth and f_{c} is the the "attenuation bandwidth," called that for want of a better expression. See the information and illustrations contained in the Help  Data Entry section of the main menu. Once the order, n, is known it is possible to generate a table of values s_{i} according to the following: 
The prototype values for an nth order filter with equal input and output termination resistances is given by the following recursion formulae: 
where X_{i} is the reactance value in either Henries or Farads, and where
