Real precision in Fortran Wiki

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
{: lang=fortran }
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)
{: lang=fortran }
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))
{: lang=fortran }
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)
{: lang=fortran }
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
{: lang=fortran }
## External Links
* [how to declare doubles in f95](http://groups.google.com/group/comp.lang.fortran/browse_thread/thread/932cb33652397d9e/)
on [[comp lang fortran|comp.lang.fortran]] (28 Jan 2009).
* [example of KIND to replace DOUBLE PRECISION ](http://groups.google.com/group/comp.lang.fortran/browse_thread/thread/cb6d2ad659d7ce3b)
on [[comp lang fortran|comp.lang.fortran]] (8 May 2009).
## References
* Chin, L. S., D. J. Worth and C. Greenough (2006).
[Thoughts on using the Features of Fortran 95](http://www.softeng.rl.ac.uk/st/archive/SoftEng/SESP/Publications/FortranDosDont/FortranDosDont.pdf).
Software Engineering Group Notes no. SEG-N-003.
* Metcalf, M., J. Reid, and M. Cohen (2004).
[Fortran 95/2003 Explained](http://www.amazon.com/exec/obidos/ASIN/0198526938/fortran-wiki-20).
Oxford University Press.
* Lemmon, D. R. and J. L. Schafer. (2005).
[Developing Statistical Software in Fortran 95](http://www.amazon.com/exec/obidos/ASIN/0387238174/fortran-wiki-20).
Springer.