I spent my first 200 hours in Factorio convinced balancers were a myth. Belts got uneven. Sides of my bus ran empty while the other side backed up. I built more smelting columns instead of fixing the root cause -- my belts were unbalanced. Here's the balancer pattern that fixed everything.
Why Balancers Matter
In a main bus factory, production lines pull from specific lanes. If your bus has 4 lanes of iron plates and three lanes are idle while one lane handles everything, you get:
- Production starves because it only pulls from the active lane
- Belts jam because the other three lanes can't empty
- Furnace arrays back up because their output belts are full
A balancer takes N input belts and distributes the items evenly across N output belts. Any combination of inputs and outputs works evenly.
The 4x4 Throughput-Unlimited Balancer
The standard 4x4 balancer uses the "blessed" splitter arrangement: 3 columns of splitters with underground belt crossings on the middle rows.
| Layer | Splitter pair | What it does |
|---|---|---|
| Layer 1 | S1+S2 | Split belts 1-2 and 3-4 into pairs |
| Layer 1 | S3 | Cross-balance belt 2 with belt 3 |
| Layer 2 | S5+S6 | Cross-balance upper and lower belt pairs |
| Layer 3 | S7+S8 | Final merge into 4 balanced outputs |
The splitters count from left to right. S1 takes belts 1 and 2; S2 takes belts 3 and 4; S3 sits offset to cross belt 2 (from S1's bottom) with belt 3 (from S2's top). An underground belt carries the crossing lane in each case.
Lane Balancers
A full 4x4 belt balancer balances belts but not individual lanes. If your bus uses both lanes of each belt independently, you also need lane balancers.
A lane balancer uses 2 splitters in a loop: each splitter takes one lane from each of two input belts and outputs to two lanes on one belt. The pattern:
- Splitter A: takes left lane of belt 1 and right lane of belt 2 => outputs to belt A
- Splitter B: takes right lane of belt 1 and left lane of belt 2 => outputs to belt B
Place a lane balancer just after your smelting column output, before the belts enter the main bus. This ensures both lanes of each bus belt carry equal material.
Common Mistakes
Overbuilding balancers. You do not need an 8x8 balancer for a 4-belt bus. Two 4x4 balancers in parallel with a shared return belt work identically and cost less space. An 8x8 balancer needs 16 splitters; two 4x4s need 12.
Forgetting throughput limits. A balancer that is not throughput-unlimited will jam if some output belts back up. The 3-layer design above is throughput-unlimited -- it maintains full throughput even with 1, 2, or 3 outputs blocked.
Balancing too often. Place one balancer per smelting column output, and at most one midpoint balancer. Every balancer adds belt length and latency. Three balancers on a bus segment creates more problems than it solves.
The Simple 2x2 Mini Balancer
For two-belt setups (early game or small sub-factories), a 2x2 balancer is just one splitter. Feed two input belts into one splitter, take both output belts. It balances perfectly with zero underground belt crossings.
Underground Belt Routing
The 4x4 balancer requires underground belt crossings between the first and second splitter columns. Here is the specific routing:
- From S1 (top): top output goes straight to S5; bottom output goes through an underground belt under the S3 middle crossing, then surfaces to reach S5
- From S2 (bottom): bottom output goes straight to S6; top output routes up through an underground belt under S3
- From S3 (middle): left output crosses up to feed S5; right output crosses down to feed S6
If you build the splitters 2 tiles apart (the minimum gap), the underground belts are exactly 4 tiles long. Place the underground exits between the splitter columns.
Bottom Line
Balancers are one of those Factorio mechanics that look complicated until you build them once. The 4x4 throughput-unlimited balancer is the single most useful blueprint for any main bus factory. Lane balancers handle the rest. Build them once, stamp them into every smelting column output, and never think about belt imbalance again.
Numbers to remember:
- 6 splitters for a 4x4 throughput-unlimited balancer
- 2 splitters per lane balancer pair
- Place at smelting column output and at bus midpoint
- 8x8 balancers are overkill -- parallel 4x4s work better
Previous: Main Bus Guide Next: Mall Guide
Community Verification & Resources
- Factorio Wiki -- Transport Belts -- belt speed and mechanics
- Reddit -- Balancer Book -- compact balancer blueprints from the community
- Factorio Prints -- Balancers -- community-shared balancer designs
Related: Main Bus Guide | Mall Guide | Megabase Transition Guide