What is line balancing?
Line balancing spreads the work of a production line evenly across workstations so the line can meet takt time with the fewest stations and the least idle time. An unbalanced line has one overloaded station that sets the pace for everyone (the bottleneck) and others that wait. Balancing is needed when a line is designed, when demand changes and takt time moves, and when products or methods change.
Line balancing formulas
| Metric | Formula |
|---|---|
| Theoretical minimum stations | ⌈ Σ task times ÷ takt time ⌉ |
| Line efficiency | Σ task times ÷ (stations × takt time) |
| Balance efficiency | Σ task times ÷ (stations × bottleneck station time) |
| Balance delay | 1 − balance efficiency |
| Smoothness index | √ Σ (bottleneck time − station time)² |
| Positional weight | Task time + times of all tasks that follow it |
Line balancing example
An office chair line has eleven tasks totalling 145 seconds and a takt time of 45 seconds, so at least four stations are needed. Precedence matters: the seat must be mounted on the gas lift before the backrest and armrests go on, and the function test needs the casters and the torque check. Auto-balancing by positional weight assigns the tasks to four stations, all within takt. The Yamazumi chart shows how close each station is to takt, so you can see where a small change in method would free time.
Tips for a better balance
- Use measured times from a time study, not estimates.
- Balance to planned cycle time (e.g. 90% of takt) to leave room for normal losses.
- Remove waste before balancing: balancing waste just spreads it around. See waste analysis.
- Challenge precedence: many constraints come from habit, not physics.
- Document the result as standard work for each station.
How to use this Line Balancing tool
- Set takt time. Enter the takt time from customer demand (use the takt time calculator if needed).
- List the tasks. Add each task with a code, description and time in seconds.
- Define precedence. For each task, list the codes of the tasks that must be finished first (e.g. "A, C").
- Auto-balance. Click Auto-balance the line: tasks are assigned to stations by ranked positional weight within takt.
- Fine-tune on the chart. Click any block on the Yamazumi chart to change its time or move it to another station. Precedence violations are flagged.
- Compare and export. Check line efficiency, balance efficiency and the smoothness index, then export the balance to PDF or Excel.
Frequently asked questions
What is line balancing?
Line balancing is assigning the tasks of an assembly or production line to workstations so that each station has about the same amount of work, no station exceeds takt time, and the number of stations is as small as possible, while respecting the order in which tasks must be done.
How do you calculate the minimum number of stations?
The theoretical minimum number of workstations is the total work content divided by takt time, rounded up. For 145 seconds of work and a 45-second takt time, at least ceil(145 ÷ 45) = 4 stations are needed. Precedence constraints and task sizes may force more.
What is a Yamazumi chart?
A Yamazumi chart (Japanese for "stacked up") shows the work content of each station or operator as a stacked bar, one block per task, with a horizontal line at takt time. It makes imbalance, overloaded stations and idle time visible at a glance and is the standard visual for rebalancing a line.
How is line efficiency calculated?
Line efficiency = total work content ÷ (number of stations × takt time). Balance efficiency uses the bottleneck station time instead of takt. Balance delay is 1 minus balance efficiency, and the smoothness index is the square root of the sum of squared differences between the bottleneck time and each station time.
What is the ranked positional weight method?
The ranked positional weight (RPW, or Helgeson–Birnie) method gives each task a weight equal to its own time plus the time of all tasks that follow it. Tasks are assigned in order of decreasing weight to the first station where they fit within takt and all their predecessors are already assigned. It is a simple heuristic that usually gives good balances.
What if one task takes longer than takt?
A task longer than takt cannot fit in one station. Split it into smaller tasks, run two stations in parallel for it, reduce its time with better methods or tooling, or automate it.