MANPOWER - Going beyond military equipment totals and perceived fighting strength is the actual manpower that drives a given military. Wars of attrition favor those with more.
LAND SYSTEMS - Tank value includes Main Battle Tanks, light tanks and tank destroyers, either wheeled or tracked. AFV value includes Armored Personnel Carriers (APCs) and Infantry Fighting Vehicles (IFVs).
NAVAL POWER - Aircraft Carrier value includes dedicated "helicopter carrier" vessels. Total naval strength includes all known auxiliaries as well.
RESOURCES (PETROLEUM) - Despite the advances made in battlefield technology, oil remains the lifeblood of any fighting force and supporting economy.
LOGISTICAL - War is as much a battle of logistics - moving man and machine from / to points all over - as it is direct combat. Labor Force adds to available wartime industry strength.
FINANCIAL (in USD) - Regardless of strength in numbers, war is still driven by financing as much as any one leader or weapon.
GEOGRAPHY (in km) - Geographical values primarily figure into a defensive-minded war (i.e. invasion).
Sources: CIA.gov, CIA World Factbook, wikipedia.com, public domain print and media sources and user contributions. Some values may be estimated when official sources are lacking.
The values showcased above are all considered for the final GFP ranking recognized as the "Power Index" (abbrv: "PwrIndx"). PwrIndx scores are judged against a perfect value of "0.0000" which is realistically unattainable due to the number of factors considered per country. Balance is the key - a large, strong fighting force across land, sea and air backed by a resilient economy and defensible territory along with an efficient infrastructure - such qualities are those used to round out a particular nation's total fighting strength on paper; it is not enough to field 10 million men or 20,000 tanks or lead the world in oil production.
Each nation's final ranking also carries with it bonuses and penalties as needed while landlocked countries (ex: Austria) are not penalized for lack of a standing navy - though they do suffer a penalty for not maintaining a viable merchant marine force.
It bears repeating that nuclear weapons are NOT taken into account for this listing. Powers are based on conventional weaponry in inventory as well as other factors such as economic strength, border size, self-sustainability (in regards to wartime resources such as oil), water access etc...
A total of 126 countries currently make up the GFP ranked list.