<PAGE>   1
                                                                    EXHIBIT 12.1


                            PAGEMART WIRELESS, INC.
               COMPUTATION OF RATIO OF EARNINGS TO FIXED CHARGES
                                 (IN THOUSANDS)
                                  (UNAUDITED)

<TABLE>
<CAPTION>


                                        YEAR ENDED     YEAR ENDED      YEAR ENDED      YEAR ENDED
                                       DECEMBER 31,   DECEMBER 31,     DECEMBER 31,    DECEMBER 31,
                                          1994            1995            1996            1997
                                      ------------    ------------    ------------    ------------

<S>                                   <C>             <C>             <C>             <C>
Earnings:
  Net loss ........................   $    (45,813)   $    (53,113)   $    (48,598)   $    (43,887)
Add: Amount of
   previously capitalized
   interest amortized .............             --              --              --              --
Add: Fixed charges ................         12,933          30,720          35,041          38,499
                                      ------------    ------------    ------------    ------------
Adjusted earnings .................        (32,880)        (22,393)        (13,557)         (5,388)

Fixed charges:
  Interest in
    indebtedness ..................         11,209          28,383          32,368          36,672
  Amortization of debt
    issuance costs ................            800           1,052           2,143             844
  Interest portion
    of rental and
    lease expense .................            924           1,285             530             983
                                      ------------    ------------    ------------    ------------
Fixed charges .....................         12,933          30,720          35,041          38,499
Deficiency of
    earnings available to
    cover fixed charges ...........   $    (45,813)   $    (53,113)   $    (48,598)   $    (43,887)
                                      ============    ============    ============    ============

Earnings to fixed charges ratio ...             --              --              --              --


<CAPTION>

                                                         THREE MONTHS    NINE MONTHS
                                         YEAR ENDED         ENDED           ENDED
                                         DECEMBER 31,      SEPT. 30,      SEPT. 30,
                                            1998(1)          1999           1999
                                         ------------    ------------    ------------

<S>                                      <C>             <C>             <C>
Earnings:
  Net loss ........................      $    (59,316)   $    (24,243)   $    (74,794)
Add: Amount of
   previously capitalized
   interest amortized .............               252             572           1,716
Add: Fixed charges ................            43,798          16,722          48,177
                                         ------------    ------------    ------------
Adjusted earnings .................           (15,266)         (6,949)        (24,901)

Fixed charges:
  Interest in
    indebtedness ..................            52,645          16,327          46,845
  Amortization of debt
    issuance costs ................             1,172             327             960
  Interest portion
    of rental and
    lease expense .................             1,428              68             372
                                         ------------    ------------    ------------
Fixed charges .....................            55,245          16,722          48,177
Deficiency of
    earnings available to
    cover fixed charges ...........      $    (70,511)   $    (23,671)   $    (73,078)
                                         ============    ============    ============

Earnings to fixed charges ratio ...                --              --              --
</TABLE>





(1) The fixed charge adjustment to earnings excludes $11.4 million of
    capitalized interest for the year ended December 31, 1998.