Two countries with the same number of factories can produce wildly different amounts of equipment, and production efficiency is most of the reason. It is the multiplier that grows while a line runs and collapses when you change your mind. This guide covers how it builds, what keeps it across a switch, and when converting beats replacing.
Quick answer
| What | Answer |
|---|---|
| What efficiency does | Multiplies the output of a production line without adding factories |
| Where it starts | Low on a new line, and it climbs while the line runs |
| What resets it | Switching a line to different equipment |
| Retention | Keeps part of the efficiency across a switch — the reason to research it |
| Conversion | Turns old equipment into new without starting from zero |
| The mistake | Rebuilding lines constantly for a better design, and never letting one mature |
How a production line works
For production by Civilian factories , see Construction Production of military and naval equipment is organized into production lines, each of which produces a single type of equipment at a time using assigned Military factories for land- and air-based equipment and Naval dockyards for naval equipment. Up to 150 factories may be assigned to a single production line, but a country can operate multiple lines producing the same item. Ship production lines have a maximum number of dockyards that can be assigned.
Production efficiency
Loss in production with respect to the fraction of starting production efficiency versus the production efficiency cap. This is measured in factory-days relative to running at production efficiency cap for the entire period. Each production line has its own efficiency level which determines how good it is at producing its current equipment . The higher the efficiency, the faster the production line produces equipment, up to a maximum value. Running the same production line continuously gradually increases its efficiency.
Retention: what survives a switch
Switching a production line to a related type of equipment not only cancels production of the previous unit and the time spent on it, it also reduces the line's production efficiency. The decrease depends on how drastic the change is, with the player getting to keep a greater percentage of the line's previous efficiency based on how drastic the change is: Retain 90% : Different variant of the same model (e.g. G [variants of the German medium tank 1], this also includes switching from license production to own production of the same equipment type, e.g. Italian Fighter II C.202 Folgore to German Fighter II FW190) Retain 70% : Different model of the same chassis (e.g.
A new tank design is not automatically worth switching to. A line that has been running for two years carries an efficiency a fresh line will take just as long to rebuild, and retention only returns part of it. The question is never whether the new design is better — it is whether it is better by more than the efficiency you are about to throw away.
Equipment conversion
A production line can be set to produce an item by converting obsolete or foreign (captured, lend-leased , license-built, or purchased ) equipment of the same chassis. This can be done at any time (assuming input items are available) by clicking the button in the bottom-left of the production line's box, and incurs no loss of production efficiency. When the line runs out of equipment to convert it will automatically revert to production from raw materials. Equipment conversion can be used for things such as converting tanks to tank destroyers, or upgrading old tank and aircraft designs with new modules.
Resources and what they cost you
In addition to acquiring more resources when there is a shortage, there are many ways to reduce how many are consumed by a nation's production lines: Lower-tier equipment, components, and chassis often have reduced resource costs. Aircraft designs with Non-Strategic Material Usage have reduced Aluminum costs. Omitting resource-costing components from tank , ship , and aircraft designs like Squeeze-Bore Adaptor , Flight Deck Armor , or Self Sealing Fuel Tanks reduces their resource costs.
Also worth knowing
- Licences are a shortcut with a tax. Producing another country's design costs you efficiency and gives them research, which is a real trade rather than free equipment.
- Ships are their own case. Naval production does not behave like the rest and the wiki keeps it in a separate section for that reason.
Source: Production on the Hearts of Iron 4 Wiki.
Frequently asked
What is production efficiency in HOI4?
It is a multiplier on a production line's output that grows the longer the line runs the same equipment. Two countries with identical factories produce very different amounts if one keeps its lines and the other keeps changing them.
Does switching equipment reset production efficiency?
Switching a line to different equipment drops its efficiency. Retention, which is researched, keeps part of it across the change — which is why the research matters more than it looks.
Should I switch to a better tank design?
Only if it is better by more than the efficiency you lose. A line running for years carries progress a new line has to rebuild from the start, and retention returns only part of it.
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