The limitations of selected run length limited codes relative to their practical recording density at given error rates are examined with a known head/disc system interface. A method is presented for the evaluation of run length limited codes on the basis of error rate as a function of linear density. Experimental measurements of the intrinsic error rate as a function of the linear density were utilized with theoretical characteristics of the code to determine the practical data density limitations. Depending upon the system configuration, the effectiveness of the code can be limited either by the noise characteristics and/or pattern induced peak shift. A 33 percent increase in linear density at an error rate of 10-10can be obtained by the proper choice of code relative to MFM without changing the head/media/system interface.