From http://www.chem.ox.ac.uk/fortran/subprograms.html

main.f90

PROGRAM SUBDEM
    REAL A,B,C,SUM,SUMSQ
    CALL INPUT( + A,B,C)
    CALL CALC(A,B,C,SUM,SUMSQ)
    CALL OUTPUT(SUM,SUMSQ)
END

subroutines.f90

SUBROUTINE INPUT(X, Y, Z)
    REAL X,Y,Z
    PRINT *,'ENTER THREE NUMBERS => '
    READ *,X,Y,Z
    RETURN
END

SUBROUTINE CALC(A,B,C, SUM,SUMSQ)
    REAL A,B,C,SUM,SUMSQ
    SUM = A + B + C
    SUMSQ = SUM **2
    RETURN
END

SUBROUTINE OUTPUT(SUM,SUMSQ)
    REAL SUM, SUMSQ
    PRINT *,'The sum of the numbers you entered are: ',SUM
    PRINT *,'And the square of the sum is:',SUMSQ
    RETURN
END