A country's manpower denotes the number of reserves that can be drafted into military service. This draft happens either through the raising of new regiments, or by reinforcements to existing regiments. Manpower is recruited from your provinces, and thus the most effective way to improve your manpower is to gain more provinces - be it through conquest, diplomacy or colonization.
This is the amount of manpower you gain each year from that province.
province manpower = int((125*Units)*baseMP + buildings)*(%modifiers)
where Units is a number derived from population (and, starting in v3.1, base tax) and baseMP is the province's manpower value in the province history file which is visible in-game as a modifier to the garrison growth rate. The Armory, Training Fields, Barracks and Conscription Center buildings give bonuses of 250, 250, 1000 and 5000, respectively, that are added to the base manpower before modifiers are applied.
Some 'global' %modifiers:
- The Feudal Monarchy form of government gives a 10% bonus
- The Tribal Despotism form of government gives a 20% bonus
- The Steppe Nomads form of government gives a 100% bonus.
- The Revolutionary Empire form of government gives a 25% bonus.
- The National Conscripts national idea gives a 50% bonus
- The Land vs. Naval domestic policies slider adds 5% for each step to the left of the middle, retracts 5% for each step to the right.
- The Quality vs. Quantity domestic policies slider retracts 2.5% for each step to the left of the middle, adds 2.5% for each step to the right.
- Catholic countries at war with the Crusade target get a +30% bonus.
- Members of the Holy Roman Empire (but not the emperor) get a +10% bonus.
In addition to the above, the difficulty levels gives additional modifiers: Human-played country:
- +50% on very easy
- +25% on easy
- -10% on very easy
- -5% on easy
- +25% on hard
- +50% on very hard
There are also some modifiers which only apply to individual provinces:
- +20% for a Regimental Camp building
- +30% for an Arsenal building
- +10% for a capital city.
- +10% if affected by a National Focus
- +10% per level for a Center of Trade.
- -30% non-accepted culture
- -50% overseas (i.e. no land connection to capital and on different continent from capital)
All applicable modifiers are added together before multiplying by the other factors in the equation.
Holy Roman Emperor bonus
Finally, if you are the Holy Roman Emperor it gives you the following
- +500 manpower for each member state in the Holy Roman Empire. For example, if there are 10 member states in the Holy Roman Empire you will get an additional 5000 added to your yearly manpower.
Max manpower pool
The maximum size of your manpower pool is ten (two for all previous versions) times the total of your yearly manpower. For the modders: This factor is defined in defines.txt as #_MDEF_MAX_MANPOWER_.