<PAGE>
 
                                                                      EXHIBIT 11

               CHAMPION INTERNATIONAL CORPORATION AND SUBSIDIARIES

           Calculation of Primary Earnings (Loss) Per Common Share and
           Fully Diluted Earnings (Loss) Per Common Share (unaudited)
                        (in thousands, except per share)

<TABLE>
<CAPTION>
                                                                          Nine Months Ended               Three Months Ended
                                                                            September 30,                   September 30,
                                                                  ----------------------------------   -----------------------------
                                                                       1997              1996              1997              1996
                                                                  ---------------   ----------------   --------------   ------------

<S>                                                               <C>               <C>                <C>              <C>     
Primary earnings (loss) per common share:
  Net Income (Loss) Applicable to Common Stock                          $(28,275)         $131,202         $ 20,166         $ 32,001
                                                                        ========          ========         ========         ========


  Average number of common shares outstanding                             95,718            95,515           95,903           95,529
                                                                        ========          ========         ========         ========


  Per share                                                             $   (.30)         $   1.37         $    .21         $    .33
                                                                        ========          ========         ========         ========


Fully diluted earnings (loss) per common share:
  Net Income (Loss) Applicable to Common Stock                          $(28,275)         $131,202         $ 20,166         $ 32,001


  Add income effect, assuming conversion of
    dilutive convertible securities                                         --                --               --               --
                                                                        --------          --------         --------         --------


  Net income (loss) on a fully diluted basis                            $(28,275)         $131,202         $ 20,166         $ 32,001
                                                                        ========          ========         ========         ========


  Average number of common shares outstanding                             95,718            95,515           95,903           95,529


  Add common share effect, assuming conversion
    of dilutive convertible securities                                      --                --               --               --
                                                                        --------          --------         --------         --------

  Average number of common shares outstanding
    on a fully diluted basis                                              95,718            95,515           95,903           95,529
                                                                        ========          ========         ========         ========


  Per share                                                             $   (.30)         $   1.37         $    .21         $    .33
                                                                        ========          ========         ========         ========

</TABLE>

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      NOTE:

     (1) The computation of fully diluted earnings per common share assumes that
         the average number of common shares outstanding during the period is
         increased by the conversion of securities having a dilutive effect, and
         that net income applicable to common stock is increased by dividends
         and after-tax interest on such securities.