Fortran Wiki
Real precision

In Fortran 90 and later, intrinsic types such as real and integer have a kind attribute which guarantees a specific precision and/or range. real*8 and counterparts should no longer be used (Chin, Worth, and Greenough, 2006, p. 5). double precision is also no longer needed and can be thought of as a kind of real. The following example declares two real variables a and b which are guaranteed to have at least 15 significant digits of precision and an exponent range of at least 307 (Lemmon and Schafer, 2005, p. 20–22):

integer, parameter :: dp = selected_real_kind(15, 307)
real(kind=dp) :: a
real(dp)      :: b

One can also guarantee precision up to that of the machine-compiler-specific double precision real by using the kind function to get the kind of 1.d0:

integer, parameter :: dp = kind(1.d0)

Another possibility, suggested by Metcalf, et. al (2004, p. 71), ensures that the double and quad types are actually twice and four times the precision of a single:

integer, parameter ::                              &
    sp = kind(1.0),                                &
    dp = selected_real_kind(2*precision(1.0_sp)),  &
    qp = selected_real_kind(2*precision(1.0_dp))

Finally, precision of 32-, 64-, and 128-bit reals can usually be obtained with the following constants:

integer, parameter :: sp = selected_real_kind(6, 37)
integer, parameter :: dp = selected_real_kind(15, 307)
integer, parameter :: qp = selected_real_kind(33, 4931)

In Fortran 2008, also the following constants are available:

use, intrinsic :: iso_fortran_env
integer, parameter :: sp = REAL32
integer, parameter :: dp = REAL64
integer, parameter :: qp = REAL128

References