How to calculate the PostgreSQL database size

Question : How can I calculate the size of a PostgreSQL database using psql?I want to use the command line and execute a sql statement for various checks?

Answer: An easy way to derive the Postgresql database size is to use the    pg_database_size() function. The   pg_database_size() reads the database name and returns the database size in bytes. 

For example : SELECT pg_database_size('my_db_name')

This returns in bytes - which unless you enjoy calculating byte sizes - there is another function you can use which will make these numbers human readable - pg_size_pretty()

Combining these two functions will return the database size in a a human readable method -

SELECT pg_size_pretty( pg_database_size('my_db_name') );