<PAGE>
 
                                                                      EXHIBIT 11

                  PLATINUM technology, inc. and subsidiaries
                     Basic and Diluted Earnings Per Share
                   (In thousands, except per share amounts)

<TABLE> 
<CAPTION> 
                                                                 Three Months Ended            Nine Months Ended
                                                                   September 30,                 September 30,
                                                                  1998        1997              1998       1997
                                                              ------------------------------------------------------
<S>                                                           <C>           <C>            <C>           <C> 
Basic income (loss) per share:
     Net income (loss)                                         $  19,385    $  14,018      $  (35,157)   $  (85,846)
                                                              ------------------------------------------------------
     Weighted-average common shares outstanding                   85,568       78,253          82,965        77,457
                                                              ------------------------------------------------------
        Per share amount                                       $    0.23    $    0.18      $    (0.42)   $    (1.11)
                                                              ======================================================

Diluted income (loss) per share:
     Net income (loss)                                         $  19,385    $  14,018      $  (35,157)   $  (85,846)
     Add back of tax-effected interest expense(1)                  1,358          -               -             -
                                                              ------------------------------------------------------
                                                               $  20,743    $  14,018      $  (35,157)   $  (85,846)
                                                              ------------------------------------------------------

     Weighted-average common shares outstanding                   85,568       78,253          82,965        77,457

     Add:   Effect of dilutive securities:
            Stock options                                          5,517        4,184             -             -
            Convertible preferred stock                            1,768          -               -             -
            Convertible subordinated notes (1)                     8,240          -               -             -
                                                              ------------------------------------------------------

     Diluted weighted average common shares outstanding          101,093       82,437          82,965        77,457
                                                              ------------------------------------------------------
        Per share amount                                       $    0.21    $    0.17      $    (0.42)   $    (1.11)
                                                              ======================================================
</TABLE> 

(1)  The dilutive effect of the convertible subordinated notes due November 15,
     2001 (the "1996 Notes") is determined by using the "if-converted" method.
     For the third quarter of 1998, the 1996 Notes have a dilutive effect on
     earnings per share. To reflect the assumed conversion of the 1996 Notes,
     approximately 8,240,000 shares were included as common stock equivalents
     and approximately $1,358,000 of tax-effected interest expense was added
     back to net income.