For modern Fortran development, “.f90” is the recommended filename suffix. For modern Fortran code that needs to be preprocessed (e.g. with #ifdef statements) then “.F90” is the recommended filename suffix.

For legacy Fortran 77 code, “.f” is the recommended filename suffix. For Fortran 77 code that needs to be preprocessed, “.F” is the recommended filename suffix.

For a time some had suggested other filename extensions, but being as Fortran standard only recommends the latest version, by default we use the first “free-form” syntax Fortran 90 standard “.f90” suffix to indicate the current Fortran standard. This helps avoid too many ambiguous file suffixes.

C and C++ likewise do not indicate their language standard year in the file suffix.