Order of filter is very important as far as the characteristics of filter design is concerned. This particular GUI is for all Butterworth IIR filters. Order of butterworth filter is given by :
where Ap and As are passband and stopband attenuations in dB and wsp is the prototype stopband edge frequency of digital filter and wpp is the prototype passband edge frequency i-e cutoff of frequency of the digital filter to be designed.
You may calculate from the above formula but as far as designing of the problems for student is concerned, it may take a long time to take correct values out of the calculator. There you need an easy interface to get your work done. This assignment was given to us by our Professor, S K Hasnain who is the author of two books on Signals and Systems and Digital Signal Processing. Below you can download the M-File and Fig File. But if you dont have the basic idea about Graphical User Interface, see how to make a simple calculator in MATLAB GUI.