<PAGE>
 
                                                                      EXHIBIT 11

                            ACT MANUFACTURING, INC.


               COMPUTATION OF NET INCOME (LOSS) PER COMMON SHARE

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

<TABLE>
<CAPTION>
                                                                           THREE MONTHS ENDED
                                                                       --------------------------
                                                                           1998          1997
                                                                       ------------  ------------
<S>                                                                    <C>           <C>
BASIC NET INCOME (LOSS) PER COMMON SHARE:
  Net income (loss) as reported....................................       $  796        $ (620)
  Weighted average number of common shares outstanding:
     Common Stock..................................................        9,063         9,050
                                                                          ------        ------

     Basic net income (loss) per common share......................       $ 0.09        $ (.07)
                                                                          ======        ======
 
DILUTED NET INCOME (LOSS) PER COMMON SHARE:
  Net income (loss) as reported....................................       $  796        $ (620)
  Weighted average number of common shares outstanding:
     Common Stock..................................................        9,063         9,050
     Effect of stock options.......................................           93             -
                                                                          ------        ------
        Total......................................................        9,156         9,050
                                                                          ------        ------
     Diluted net income (loss) per common share....................       $ 0.09        $ (.07)
                                                                          ======        ======
 
<CAPTION>
                                                                            NINE MONTHS ENDED
                                                                       --------------------------
                                                                           1998          1997
                                                                       ------------  ------------
BASIC NET INCOME (LOSS) PER COMMON SHARE:
  Net income (loss) as reported.....................................      $ (244)       $6,373
  Weighted average number of common shares outstanding:
     Common Stock...................................................       9,063         8,853
                                                                          ------        ------
 
     Basic net income (loss) per common share.......................      $(0.03)       $  .72
                                                                          ======        ======
                                                                          
DILUTED NET INCOME (LOSS) PER COMMON SHARE:
  Net income (loss) as reported.....................................      $ (244)       $6,373
  Weighted average number of common shares outstanding:
     Common Stock...................................................       9,063         8,853
     Effect of stock options........................................          --           322
                                                                          ------        ------
        Total.......................................................       9,063         9,175
                                                                          ------        ------
     Diluted net income (loss) per common share.....................      $(0.03)       $  .69
                                                                          ======        ======
</TABLE>