A lot of different algorythms have been proposed in order to calculate the global power of a nation. Some of them take into account a lot of parametres and are quite difficult to manage.
The Indian economist Arvind Virmani has proposed a simplified way to calculate the national power potential NPP as
NPP = P * y^a
- P is the population
- y is the pro capite GDP at purchasing power parity
- b is a coeeficient whose estimate value is 1.5 to 1.6
His paper can be found at http://icrier.org/pdf/WP175VIPP8.pdf