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---
title: constants
---
[TOC]
## Introduction
The [[stdlib_constants]] module provides mathematical constants and the most common physical constants.
**Warning**: The names of the most common physical constants are kept short as they are inside a dedicated module.
Nonetheless, in case of overlapping names, they can always be renamed as following:
```fortran
use stdlib_constants, only: clight => c
```
## Codata
The [[stdlib_codata(module)]] module defines all codata (physical) constants as derived
type. The module is automatically generated with a simple
[parser written in Python](https://github.com/MilanSkocic/codata/)
The latest codata constants were released in 2022 by the [NIST](http://physics.nist.gov/constants)
All values for the codata constants are provided as double precision reals.
The names are quite long and can be aliased with shorter names.
The derived type [[stdlib_codata_type(module):codata_constant_type(type)]] defines:
* 4 members:
* `name` (string)
* `value` (double precision real)
* `uncertainty` (double precision real)
* `unit` (string)
* 2 type-bound procedures:
* `print`: to print the values of the constant members;
* `to_real`: to get the value or the uncertainty to the desired precision.
A module level interface [[stdlib_codata_type(module):to_real(interface)]] is
available for getting the constant value or uncertainty of a constant.
## `to_real` - Get the constant value or its uncertainty.
### Status
Experimental
### Description
Convert a [[stdlib_codata_type(module):codata_constant_type(type)]] to a `real` (at least `sp`, or `dp`) scalar.
**Warning**: Some constants cannot be converted to single precision `sp` reals due to the value of the exponents.
### Syntax
`r = ` [[stdlib_codata_type(module):to_real(interface)]] `(c, mold [, uncertainty])`
### Arguments
`c`: argument has `intent(in) ` and shall be of type [[stdlib_codata_type(module):codata_constant_type(type)]].
`mold`: argument has `intent(in)` and shall be of `real` type.
**Note**: The type of the `mold` argument defines the type of the result.
`uncertainty` (optional): argument has `intent(in)` and shall be of `logical` type.
It specifies if the uncertainty needs to be returned instead of the value. Default to `.false.`.
### Return value
Returns a scalar of `real` type which is either the value or the uncertainty of a codata constant.
## Example
```fortran
{!example/constants/example_constants.f90!}
```
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