Technology

Research is used to unlock technologies.

Research slots
Technology is unlocked by assigning a research slot to it for the required number of days. Most countries start with 3 research slots. However, there are some exceptions, such as the Barrad Magocracy which only starts with 2.

Each research slot can store up to 30 points of Days Saved without being assigned to a specific tech. Days Saved functions as days added to research progress once a tech is chosen for the slot.

Research time
Each technology has a base time cost. They are multiples of the standard base cost of 100 days. (Table WIP, so far only vanilla techs are represented here at vanilla values)

Research speed
The base research cost only represents an abstract number of days the research will take. In practice, due to modifiers, research will progress by more or less than exactly one research-cost-day per day. Research speed modifiers stack additively to research speed (instead of subtractively to research cost). For example, 106 days base cost at 15% research speed bonus will take (106 over 1+15%) days, which is 93 days.

Before patch 1.6 for HoI4 research bonuses and maluses where calculated differently. A modifiers is automatically applied to research speed to get it maluses closer to pre-1.6 values

Check here for a plotted out graph of the calculations

Days Saved are subtracted from the modified expected number of days. Thus, Days Saved benefit from (or are penalized by) the same research speed modifiers.

The aggregate research bonus is calculated and updated every day. Thus changes in research speed (except limited-use bonuses) apply to all ongoing research.

Research speed can not fall below 10% of the default speed.

Difficulty
The player can get a research speed bonus or penalty depending on the game difficulty setting as follows:
 * Civilian bonus
 * Recruit bonus
 * Regular no bonus or penalty
 * Veteran penalty
 * Elite penalty

Trade law
Trade laws can also increase research speed. The more open the trade the higher the reduction as can be seen below. Closed Economy does not give any bonus. Additionally, the economy law "Civilian Economy" gives +3% to industrial research speed and +2% to electronic research speed, while "Early Mobilization" grants +2% to industrial research.

Technologies that increase research speed
Note that some technologies that are only available to certain races or countries are not listed here.

Limited-use bonuses
Limited-use bonuses are awarded by National Focus. Once available, a limited-use research bonus is activated when a Research Slot is assigned to an applicable technology. Once activated, the limited-use bonus is tied to the technology even if the slot is re-assigned. (However, Hearts of Iron IV client is known to drop limited-use bonuses on technologies not currently assigned to a research slot upon reload. Exercise caution when activating a limited-use bonus for future use.)

A limited-use bonus can alternatively or simultaneously be an ahead-of-time bonus, which reduces the ahead-of-time penalty by the stated amount.

Equipment licensing
When licensing equipment from another country, researching that equipment gets a speed bonus of.

Technology sharing
When a country is part of a research group, it gets a research speed bonus for each other country in the group that has already unlocked that technology, up to. Tech sharing bonus functions in the same way as Limited-use bonus: once applied, the bonus is locked to the tech; changes in the country's tech sharing situation will only reflect in future research. Aside, subjects benefit less from technology sharing, depending on their autonomy level.

Other sources of research bonus
Various national ideas, like designers, theorist, and concerns continuously improve the research speed for applicable technologies while they are active. Design companies can additionally improve equipment being unlocked while they are active.

Ahead-of-time penalty
All technologies have a date attached to them when they become current. Researching them earlier comes at the cost of reduced research speed. Each year ahead adds ahead-of-time (AOT) penalty. Ahead-of-time bonuses from National Focus reduce this penalty modifier. For research ahead of time, the in-game tool-tip displays estimated time to completion. The formula can be seen at the HoI4 wiki: Ahead-of-time penalty

Below is a Python script to calculate the time it will take to research a technology, given its base cost, bonuses and AOT penalty.

Technologies
Technologies allow countries to unlock production of new units such as mechanized infantry, paratroopers, tanks and all kinds of airplanes; new equipment, from infantry weapons to carrier air groups and specialized armored units; as well as defensive and productive structures.

On the other hand, most technologies serve to improve the efficiency of current units. This can be achieved through a number of ways, such as improving the damage inflicted by the unit, its ability to penetrate the armor of armored units, its defensiveness and organization or simply its ability to detect enemy units.

It is possible to research improvements of already researched technologies. There could be small buttons on technology (e.g. possibility to operate from carriers for airframe). These improvements have to be researched separately.

Land

 * Infantry technology: Technologies that improve infantry weapons and equipment, unlock or improve motorized and mechanized units as well as special forces (marines, mountaineers and paratroopers).
 * Artillery technology: Technologies that unlock and improve all types of artillery, including anti-tank, anti-air and rocket artillery.
 * Support Companies technology: Technologies that unlock and improve all types of support battalions: Engineers, Reconnaissance, Military Police, Maintenance, Field Hospital, Logistics and Signal companies.
 * Armor technology: Technologies that unlock all types and models of tanks, from light to super heavy chassis, as well as their variants: tank destroyer, self-propelled artillery and self-propelled anti-air.

Naval

 * Naval technology: Technologies that unlock or improve all kinds of ship units: destroyers, cruisers, battlecruisers battleships, carriers, submarines or improve transports for naval invasions.

Air

 * Air technology: Technologies that unlock or improve all types of airframes: light, medium or heavy, as well as their jet propulsion versions.

Industry

 * Engineering technology: Groups all technologies related to electronic engineering (computers, radios, cryptography), rockets and atomic research.
 * Industry technology: Groups all technologies that improve production efficiency and volume of all factories, resources excavation and unlocks synthetic refineries.

Race-specific technologies
Different races allow you to research different bonuses for your nation, and unlock new units.
 * Generic race technology
 * Pony technology
 * Griffon technology
 * Deer technology
 * Barrad technology
 * Changeling technology

Development Level
Main article: Development Level

All development levels affect the research speed of a nation: the "Research base" level impacts this the most, with the worst possible science base giving a research speed debuff.