# Attrition and accidents

Attrition is the loss of equipment — not manpower — during the course of a war when fighting on the offensive. While only land units suffer from attrition, air units may suffer accidents, especially when moving in harsh weather, resulting in loss of equipment.

## Organization loss

A division loses 0.1 * attrition organization per hour from attrition both in combat and out of combat.

When out of supply, the division also suffers from an extra organization loss of 0.3 per hour scaled by out of supply progress, reaching full in 30 days.

## Equipment loss

Higher reliability reduces equipment loss from attrition.

### Land units

For land units, the average rate of equipment loss per hour for each type is

where P(loss) and P(per type loss) are respectively ATTRITION_EQUIPMENT_LOSS_CHANCE (0.1) and ATTRITION_EQUIPMENT_PER_TYPE_LOSS_CHANCE (0.1) sourced from defines.

To be specific, when ; otherwise for high reliability equipment a base attrition applies regardless of the number of equipment: .

So for 100% attrition and 0% reliability, about 12% of equipment will be lost per day. At 80% reliability, 100 days of training (enough to go from Green to Trained, or halfway from Trained to Regular) will cost about 14.4% of equipment.

Note that a loss is calculated for each type of equipment independently so the total losses of all equipment scale with the number of equipment types. This will happen if you divisions use captured equipment or several variants of the same equipment class. Also, the base attrition is relatively large for units using equipment in small quantity, e.g. support artillery company (12 artillery) and heavy self-propelled anti-air battalion (8 armor).

To minimize equipment loss from attrition, it is enough to keep reliability of variant equipment types above 1 - 10 / \text{number of equipment} . This means 0% for 10 or under, 16.7% for 12 pieces, 58% for 24 pieces, 80% for 50 pieces, 95% for 200 pieces and so on. Therefore increasing reliability through variant equipment and using maintenance companies matters the most in large divisions using expensive equipments, like tanks. Also note that normal artillery has 50% reliability while rocket artillery has 80% reliability.

### Air units

While aircraft do not suffer from attrition, they have a chance of accidents when taking off/landing. This is

or equal to 1% attrition for land units. Carrier-based aircraft suffer twice the accident rate. It can quickly turn into a major issue, chewing down through the pool of accessible planes regardless of how well the air combat goes.

### Naval units

Ships do not suffer from attrition, but the reliability corresponds with probability of getting critically hit by enemy attack.

## Methods to reduce equipment loss

There are several ways to reduce the effects of equipment loss. One way is to improve reliability by attaching a Maintenance Company to a division or creating new variants of equipment, such as tanks, ships, or planes, with increased reliability. Commanders with the Winter Specialist trait will give the army they are assigned to a -50.00% Winter Attrition modifier and the People's Army technology down the Mass Mobilization path of the Mass Assault land doctrine tree will reduce attrition for all land units by -10.0%.

### Custom difficulty settings

Adjusting the strength slider of a major country prior to a game will reduce the amount of attrition that that country's units will receive. However, doing so disables achievements.

## Footnotes

1. See in /Hearts of Iron IV/common/defines/00_defines.lua