Finding a free Fortran 77 compiler for Windows 10 (64-bit) today is straightforward, but you won't typically find a "pure" F77 download. Modern compilers are backward compatible, meaning they handle legacy files while offering 64-bit performance.
Download the installer from the MinGW-w64 project page via SourceForge. Choose the x86_64 architecture during setup for 64-bit support. fortran 77 compiler for windows 10 64-bit free download
: One of the few compilers specifically targeted at Windows that includes a personal-use free version. Finding a free Fortran 77 compiler for Windows