<PAGE>
 
                                                                    EXHIBIT 11.1

                            UNIT INSTRUMENTS, INC.
                       COMPUTATION OF EARNINGS PER SHARE
 
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<CAPTION>
                                                                                             Three Months Ended
                                                                                            -------------------
                                                                                    August 30, 1997       August 31, 1996
                                                                                    ---------------       ---------------
<S>                                                                                 <C>                   <C>
NET INCOME (LOSS)                                                                      $  359,000            $  102,000
                                                                                       ==========            ==========
 
Earnings Per Share
------------------
 
Weighted average number
of shares outstanding                                                                   4,442,681             4,384,929
 
Common share equivalents,
assuming exercise of stock
options and warrants                                                                       38,713               176,610
                                                                                       ----------            ----------

Average shares used in
computing earnings per share                                                            4,481,394             4,561,064
                                                                                       ==========            ==========
 
Net income per share                                                                   $     0.08            $     0.02
                                                                                       ==========            ==========
 
Earnings Per Share Assuming Full Dilution
-----------------------------------------

Weighted average number
of shares outstanding                                                                   4,442,681             4,384,454

Common share equivalents, assuming
exercise of stock options and                                                
warrants                                                                                  207,422               176,610
                                                                                       ----------            ----------
 
Average shares used in computing
earnings per share                                                                      4,650,103             4,561,064
                                                                                       ==========            ==========

Net income per share                                                                   $     0.08            $     0.02
                                                                                       ==========            ==========
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