Land attrition, also called supply attrition, is attrition suffered by land units due to lack of supply. As with all attrition, it is computed and applied at the first of the month. This article lays out how it is computed, so that you can learn to avoid it.
See the article on How to do Attritional Warfare for ideas on how to use attrition to your advantage.
Attrition in Brief
Here's a brief description of how attrition works. There are three numbers that must be computed to understand land attrition:
- The actual supply limit (ASL) afforded by a province; see the article on supply for how this is computed.
- The total army size (#armies), in army units, of all land forces present in the province.
- The maximum attrition level (MAL) that applies in a province. See below. The maximum attrition at high tech, in good terrain, and in supply, can be as little as 1%. It can range as high as 40% for a looted mountainous province in winter.
When ASL >= #armies, there is no supply attrition.
Only when ASL < #armies is attrition assigned. The amount of attrition is (#armies - ASL), but it is capped at a maximum of MAL.
Total Army Size
The total army size quantifies the crowdedness of the province. It is given in army units, by summing the unit weights of the land forces of each country in the province. This is every non-passenger land unit, whether friendly, enemy, or neutral; whether combat-ready or retreating or noncombatant. Each country's forces are weighted. All armies of a particular country are summed into that country's force before weighing.
If the total army size is greater the actual supply level for a particularly country, then supply attrition will be computed and applied; read on. Otherwise, no supply attrition applies, at least not to forces of the particular country. Because actual supply levels vary, it is often the case that attrition is applied to one force while another in the same province is not affected.
The maximum attrition level (MAL) for a country in a province is a cap to the amount of attrition that will be applied against forces of that country in that province. That is, actual attrition is computed as:
minimum(MAL, maximum(0, #armies - ASL))
When max attrition has been exceeded for a unit, the "attrition limit reached" bonus will appear in the tooltip. This bonus is just the difference between the attrition level that would apply absent maximum attrition, and the MAL.
Maximum attrition is computed using several factors: the country's tech level, the state of the province, and its terrain. The base of MAL is found on the following table:
Land tech Base MAL 0-5 10 6-11 9 12-17 8 18-23 7 24-29 6 30-35 5 36-41 4 42-47 3 48-43 2 54-60 1
To the base level, the following maluses based on the province are added:
- +5% - looted province
- +10% - province is out of supply for the country
- +2% - terrain is marsh
- +5% - terrain is mountains
- +8% - climate is desert
- +10% - winter in the province
Note that on the map, provinces depicted as desert (in yellow) are desert terrain, not desert climate. There is no way to see desert climate in the game.
Attrition in a province is applied in two rounds, based upon which country controls the province. All countries are divided into two groups:
- the country controlling the province and its war allies.
- all other countries
First, attrition is computed and applied to the forces of the other countries. After this is done, the attrition is recomputed using the new, lower total army size, and then applied to the group of countries with control of the province.
For each particular country, attrition is assigned to each army present individually. In an army with multiple kinds of land units, the following rules apply:
- if mixed infantry and cavalry, the infantry takes 5/6 of the attrition.
- artillery doesn't take any attrition if there are any infantry or cavalry mixed with it.
A special rule applies to armies that are purely artillery. They take only two-thirds of the attrition that they are assigned. For example, if an artillery army takes 25% computed supply attrition it will only be decreased by 16.33%.
Attrition may remove a fraction of a ship, man, or cannon without resulting in the loss of that one quantity of military. For most purposes, military units have integral quantities; a military unit with a fractional quantity of some unit type rounds up to the next highest integer. Thus, an army with 9.0000001 artillery acts like a unit of 10 artillery -- at least until a tiny amount of attrition or combat damage is imposed. The fractional losses of all military unit types can be repaired by splitting, merging, or reorganizing. For example, the unit of 9.0000001 artillery can be stopped, and split/merged, and it will have 10.0000000 artillery once again. All military units with absolute numbers less than 100 can take advantage of the fractional loss rules to avoid small amounts of attrition including movement attrition. In particular, it is very useful to extend the exploration range of ships, and useful to allow artillery movement without attrition as well as the constant movement of tiny armies.
When an army takes zero attrition, it is just that: zero. And for most armies, when X% of attrition is assigned, X% is taken. However, if a very small unit (500 men or less) is assigned any attrition at all -- that is, attrition is computed to be at least 1% -- there is a minimum level of attrition that is assigned.
The minimum attrition assigned to a unit of N men (and/or N*100 artillery) is computed using the "floor" function, as follows:
min = floor(1000/N)
In effect, this means attrition usually kills about 10 men.
You can see the table below for specific values.
#Men Min 334-500 2% 251-333 3% 201-250 4% 167-200 5% 134-166 6% 126-133 7% 112-125 8% 101-111 9% 91-100 10% 82-90 11% ... 10 100% ... 1 999%
Thus the effect of supply attrition will become drastic when armies get small. At the extreme, tiny armies (in fact every army 10 men or fewer) are destroyed by a single month of just 1% attrition!