Recommended resources
The freely available resources this lesson will direct you to.
Hello! Welcome back to our series on efficient Factorio design.
In the last two lessons, we established our fundamental unit of measurement—items per second—and quantified the maximum throughput for each of the three main belt tiers: 15, 30, and 45 items/s for yellow, red, and blue belts, respectively.
Today, we'll address a crucial question: how do we ensure our belts actually achieve that maximum throughput? The answer lies in a concept called belt compression. Understanding and mastering compression is the key to moving from a factory that simply works to one that is truly efficient and predictable.
What is Belt Compression?
At its core, belt compression is about maximizing item density. A belt is considered fully compressed when there are no empty spaces between the items it's carrying.
To get a precise definition, let's turn to the official Factorio Wiki.
Transport belts - Official Factorio Wiki
This section from the Factorio Wiki provides the formal definition of belt compression and explains how the game mechanics naturally encourage it.
Please read the two bullet points under the 'Belt properties' heading, focusing on the definition of 'Compression'.
As the wiki states, partial compression means suboptimal throughput. Let's visualize this.
The reason for this is directly tied to the throughput formula from our previous lesson:
The speed of a given belt is constant. Therefore, the only way to achieve maximum throughput is to maximize the item density. A fully compressed belt represents the maximum possible density, ensuring you get the full 15, 30, or 45 items/s you've planned for. An uncompressed belt is an underutilized asset, representing wasted logistical capacity.
A Framework for Compression: Supply and Demand
Given your background in economics, it's useful to frame belt logistics in terms of supply and demand.
- Supply: The total rate of items available from your input belts.
- Demand: The total rate of item consumption by your machines or the empty space on your output belts.
For a system to be efficient, you want your belts to remain compressed as long as supply ≥ demand. The moment items start backing up (when supply > demand), the belts should fill up completely without leaving gaps.
The following video introduces this supply/demand framework and the related concept of a "throughput unlimited" system, which is essentially a system designed to maintain compression under varying loads.
Crossbar Switches: An Alternative to Belt Balancers in Factorio
This video by Zipnotoad, while focused on an advanced alternative to balancers, provides an excellent introduction to the core distribution problem in Factorio, framing it in terms of supply and demand.
Please watch from the beginning until 04:32. Focus on how the narrator defines supply, demand, and the N-to-M distribution problem. Pay close attention to the definition of a 'throughput unlimited' balancer, as this property is exactly what ensures belt compression is maintained.
The key takeaway is that efficient belt networks must be throughput unlimited. They should not become the bottleneck themselves. If you are producing 30 iron plates per second, your red belt network should be capable of delivering all 30 plates per second to where they are needed, as long as there is demand.
Simple Techniques for Achieving Compression
Achieving compression doesn't always require complex balancers. For many common situations, simple techniques suffice. A vintage forum post from the early days of Factorio still holds some excellent advice on this.
Belts instead of trains - Factorio Forums
This classic Factorio forum post discusses fundamental techniques for maximizing belt throughput. We'll focus on two simple but powerful methods for creating and maintaining compression.
Read point 2, 'Use belt compression at the beginning,' and point 3, 'Use splitter as merger instead of feeding from side.' These describe two foundational techniques.
Let's break down the two key ideas from that post:
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Compressing with a Faster Belt: By feeding a faster belt into a slower one (e.g., a short segment of red belt feeding into a long yellow belt), the items are forced together, filling any gaps. This is a simple and effective way to ensure a belt starts out fully compressed.
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Using Splitters to Merge: A very common cause of lost compression is side-loading one belt onto another. This often leaves gaps and fails to combine the flows efficiently. The superior method is to use a splitter to merge two belts. A splitter, used in reverse, will combine two input streams, filling the gaps on the output belt far more effectively than a simple side-load.
Test your understanding!
You have two separate yellow belts coming from two small mining patches. Each belt is carrying a sparse, uncompressed stream of about 6 copper ore per second. You need to feed a line of furnaces that requires a steady 12 copper ore per second.
What is the simplest way to combine the two sparse belts into a single, compressed yellow belt that can meet the demand of the furnaces?
Show answer
The best approach is to use a splitter. By feeding the two uncompressed yellow belts into the two inputs of a splitter, the single output belt will contain a combined, compressed stream of 12 items/s. A yellow belt has a maximum capacity of 15 items/s, so it can easily handle this flow.
Side-loading one belt onto the other would be inefficient and likely fail to create a fully compressed belt, potentially starving your furnaces.
Conclusion
Mastering item flow is central to efficient factory design, and belt compression is the bedrock of that skill. Without it, your calculated ratios and planned throughputs will fail to materialize, leading to hidden bottlenecks and underperforming production lines.
Key Takeaways:
- Belt Compression means maximizing item density on a belt by eliminating gaps.
- It is essential for achieving the theoretical maximum throughput (15/30/45 items/s).
- An uncompressed belt is an underutilized asset, representing wasted logistical capacity.
- Simple techniques like merging with splitters are far more effective than side-loading for combining item streams and maintaining compression.
In our first few lessons, we've focused on the logistics: defining our unit of measure, quantifying belt capacity, and now ensuring we can use that capacity fully. In the next lesson, we'll turn our attention to the other side of the equation: the machines. You will learn to calculate the precise production and consumption rates of any machine, which will allow you to perfectly balance the supply from your assemblers with the capacity of your compressed belts.