<PAGE>
 
                                                                      EXHIBIT 11

                            ACT MANUFACTURING, INC.


               COMPUTATION OF NET INCOME (LOSS) PER COMMON SHARE

               THREE AND SIX MONTHS ENDED JUNE 30, 1998 AND 1997
                      (IN THOUSANDS EXCEPT PER SHARE DATA)
                                  (UNAUDITED)

<TABLE>
<CAPTION>
                                                           THREE MONTHS ENDED
                                                           ------------------ 
                                                            1998        1997
                                                          --------   ---------
<S>                                                      <C>        <C>
BASIC NET INCOME PER COMMON SHARE:                
    Net income as reported..........................      $    86      $3,631
    Weighted average number of common shares
      outstanding:                
        Common Stock................................        9,063       8,881
                                                          -------      ------

        Basic net income per common share...........      $  0.01      $  .41
                                                          =======      ======
                                                                       
DILUTED NET INCOME PER COMMON SHARE:                            
    Net income as reported..........................      $    86      $3,631
    Weighted average number of common shares 
      outstanding:                
        Common Stock................................        9,063       8,881
        Effect of stock options.....................          120         468
                                                          -------      ------
            Total...................................        9,183       9,349
                                                          -------      ------
        Diluted net income per common share.........      $  0.01      $  .39
                                                          =======      ======

<CAPTION>
                                                            SIX MONTHS ENDED
                                                           ------------------ 
                                                            1998        1997
                                                          --------   ---------
<S>                                                      <C>        <C>
BASIC NET INCOME (LOSS) PER COMMON SHARE:                              
    Net income (loss) as reported...................      $(1,040)     $6,993
    Weighted average number of common shares
      outstanding:                
        Common Stock................................        9,063       8,849
                                                          -------      ------

        Basic net income (loss) per common share....      $ (0.11)     $  .79
                                                          =======      ======

DILUTED NET INCOME (LOSS) PER COMMON SHARE:                            
    Net income (loss) as reported...................      $(1,040)     $6,993
    Weighted average number of common shares
      outstanding:                
        Common Stock................................        9,063       8,849
        Effect of stock options.....................           --         469
                                                          -------      ------
            Total...................................        9,063       9,318
                                                          -------      ------
        Diluted net income (loss) per common share..      $ (0.11)     $  .75
                                                          =======      ======
</TABLE>