<PAGE>
 
                                                                   EXHIBIT 12(a)


                            BANKBOSTON CORPORATION
        COMPUTATION OF CONSOLIDATED RATIO OF EARNINGS TO FIXED CHARGES

                       (Excluding Interest on Deposits)

The Corporation's ratios of earnings to fixed charges (excluding interest on
deposits) for the three months ended March 31, 1999 and 1998 and for the five
years ended December 31, 1998 were as follows:

<TABLE>
<CAPTION>
                                     Three Months Ended           
                                          March 31,                   Years Ended December 31,    
                                          ---------       ---------------------------------------------
(Dollars in millions)
                                        1999     1998     1998      1997      1996      1995      1994
                                        ----     ----     -----     -----     -----     -----     -----
<S>                                  <C>      <C>      <C>       <C>       <C>       <C>       <C> 
Net income                            $  223   $  238   $   792   $   879   $   650   $   678   $   542
Extraordinary item, net of tax                                                                        7
Income tax expense                       131      153       477       589       483       529       422
                                      ------   ------   -------   -------   -------   -------   -------
     Pretax earnings                  $  354   $  391   $ 1,269   $ 1,468   $ 1,133   $ 1,207   $   971
                                      ======   ======   =======   =======   =======   =======   =======
 
Fixed charges:
     Portion of rental expense
     (net of sublease
     rental income) which
     approximates the
     interest factor                  $   12   $   10   $    42   $    39   $    40   $    38   $    35
 
Interest on borrowed funds               264      272     1,179     1,050       873     1,079     1,038
                                      ------   ------   -------   -------   -------   -------   -------
 
          Total fixed charges         $  276   $  282   $ 1,221   $ 1,089   $   913   $ 1,117   $ 1,073
                                      ======   ======   =======   =======   =======   =======   =======
 
 
Earnings (for ratio calculation)      $  630   $  673   $ 2,490   $ 2,557   $ 2,046   $ 2,324   $ 2,044
                                      ======   ======   =======   =======   =======   =======   =======
 
 
Total fixed charges                   $  276   $  282   $ 1,221   $ 1,089   $   913   $ 1,117   $ 1,073
                                      ======   ======   =======   =======   =======   =======   =======
 
Ratio of earnings to fixed
   charges                              2.28     2.39      2.04      2.35      2.24      2.08      1.90
                                      ======   ======   =======   =======   =======   =======   =======
</TABLE>
For purposes of computing the consolidated ratio of earnings to fixed charges
"earnings" represent income before extraordinary item plus applicable income
taxes and fixed charges.  "Fixed charges" include gross interest expense
(excluding interest on deposits) and the proportion deemed representative of the
interest factor of rent expense, net of income from subleases.