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Twinworks Line Capacity Calculator
Takt time, bottleneck, OEE, throughput and Little’s law lead time of a production line.
Try an example: Three stations, one with two parallel machines, OEE losses, 400 units per 450-minute shift · Twinworks template “Line with failures & shifts” read from the model, 100 units WIP · Batch oven (24 parts per 30-minute cycle) between two cells
How it works
Analytic capacity of a serial production line: takt time from demand and available time, theoretical and effective capacity of every station with parallel machines and batch sizes, OEE = availability × performance × quality with downstream scrap, the bottleneck with and without losses, load at demand, Little’s law lead time for your work in process and the Factory Physics best-case, practical-worst-case and worst-case benchmarks. Enter stations or read them from a Twinworks model.
- Takt time = available time ÷ demand; demand rate = demand × 3600 ÷ available seconds.
- Theoretical station capacity = parallel machines × units per cycle × 3600 ÷ cycle time (units/h). The slowest station is the theoretical bottleneck; its rate rb is the line’s theoretical throughput (the same rule as the Twinworks analytic check of a deterministic serial line).
- OEE per station = availability × performance × quality (availability = run time ÷ planned time, performance = ideal cycle × units ÷ run time, quality = good ÷ processed). Effective processed rate = theoretical × A × P. Rejected units leave the line, so a station supports only (its processed rate × the product of its own and all downstream quality rates) good finished units per hour. The smallest of these is the effective throughput and the effective bottleneck.
- Little’s law: WIP = throughput × lead time, so lead time = WIP ÷ effective throughput (long-run averages, any distributions).
- Benchmarks for a given WIP w (Factory Physics): raw process time T0 = Σ cycle ÷ units per cycle, critical WIP W0 = rb × T0; best case TH = min(w ÷ T0, rb), practical worst case TH = rb × w ÷ (W0 + w − 1), worst case TH = 1 ÷ T0. A real line between best case and practical worst case is doing well; below practical worst case it has more variability than exponential.
- From a Twinworks model: stations are the machine, assembly and quality nodes from the source to the sink in flow order; cycle = mean of the cycle distribution (or the ideal cycle time with performance = ideal ÷ mean), servers = parallel machines, availability = MTBF ÷ (MTBF + MTTR) with mean times, quality = 1 − scrap %.
Analytic averages for a steady line without starvation and blocking from variability; buffers, setups, shifts and random breakdowns can lower the real output — simulate the line in Twinworks to see those effects.
Sources
- Hopp & Spearman: Factory Physics, 3rd ed. — Little’s law, critical WIP W0 = rb·T0, best case, worst case and practical worst case (Waveland Press)
- Little, J. D. C.: A proof for the queuing formula L = λW (Operations Research 9(3), 1961)
- Nakajima, S.: Introduction to TPM — Overall Equipment Effectiveness = availability × performance × quality (Productivity Press)
- Lean Enterprise Institute: Takt time = available production time ÷ customer demand (Lean Enterprise Institute)
Part of Twinworks
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Twinworks Factory digital twin & production simulation
Model a production line, simulate it and find the bottleneck — free, in your browser.
Related tools
- Twinworks Bottleneck Analysis – Find the constraint of a production line and test fixes: ranked bottlenecks with what-if gains.
- Twinworks Line Balancing – Assign assembly tasks to stations for a takt time: efficiency, balance delay and smoothness index.
- Twinworks Factory Simulation – Simulate a production line: throughput, lead time, WIP, OEE and the bottleneck, with CIs.
- Twinworks Queueing Check – M/M/1, M/M/c, M/D/1, M/G/1 and Kingman waiting times — optionally verified by simulation.
How to use the Twinworks Line Capacity Calculator
Work out takt time, the bottleneck, OEE, the real throughput of a production line and the lead time for your work in process by Little's law.
What it is for
Before you buy a second machine, promise a delivery date or start a simulation project, you want the back-of-the-envelope answer: can this line make the demand, which station holds it back, and how long does a part spend in the line? This calculator gives that answer for a serial line in a few seconds.
Enter the stations in flow order with their cycle time, the number of parallel machines, the units per cycle for batch stations, and availability, performance and quality in percent. Add the available time and the demand per period for the takt time, and the work in process for the lead time. You can also read the stations straight from a Twinworks template or model.
The results are averages. Starvation and blocking caused by variability, small buffers, setups and shift patterns are not included. For those, simulate the line in Twinworks.
Worked example
The first example has three stations: a saw at 40 s, a mill at 96 s with two machines, and a deburr station at 38 s. Each has its own losses, for example 85 % availability, 90 % performance and 98 % quality on the mill. The demand is 400 good units in a 450-minute shift, with 20 units in the line.
Effective throughput 56.2 good units/h (theoretical 75), bottleneck "Mill"; takt 67.5 s is met at 75% line OEE.
Without losses the mill makes 2 × 3600 / 96 = 75 units per hour. With its OEE losses it supports 56.23 good units per hour. The saw supports 78.8 and the deburr station 88.2, so the mill stays the bottleneck. The demand needs 53.33 units per hour, so the line copes, but the mill is loaded to 95 %. The tool warns about that: above about 85 % queues and lead times grow quickly.
With 20 units in process, Little's law gives a lead time of 21.3 minutes. The best case would be 16 minutes, the practical worst case 18.1 minutes.
How to read the result
- Effective throughput: good finished units per hour after availability, performance and quality losses. Scrap at one station also reduces what earlier stations contribute.
- Theoretical throughput: the rate of the slowest station without losses.
- Bottleneck: the station with the lowest effective capacity. A warning tells you when losses move it to another station.
- Takt time and load at demand: available time divided by demand, and each station's load at that rate.
- Line OEE and required OEE: what the line reaches and the minimum it needs for the demand.
- Little's law table: lead time = WIP ÷ throughput, compared with the best case, practical worst case and worst case.
Tips and common mistakes
- Enter cycle times per machine, not per station. Two machines at 96 s are entered as 96 s with 2 parallel machines.
- For an oven or washer that processes a batch, enter the batch time and the units per cycle.
- Quality is the share of good parts leaving a station. Rework loops are not modelled, so count reworked parts as lost capacity.
- If the WIP is below the critical WIP, even a perfect line cannot run the bottleneck at full speed. Add units or shorten the line.
- In Twinworks, Analyze → Line capacity… runs the same calculation on the open scenario and shows the last simulation run next to it.
FAQ
Why does the simulation give less than the calculator?
The calculator assumes the bottleneck never waits. Random cycle times, breakdowns and small buffers make it starve or block now and then.
What is the practical worst case?
A benchmark from Factory Physics: the throughput of a balanced line with exponential process times at your WIP. A line below it has more variability than that.
How is availability read from a model?
As MTBF ÷ (MTBF + MTTR) with the mean failure and repair times. Planned maintenance and setups are left out and listed as notes.
Can it handle parallel branches or assembly?
No. It reads serial lines only. Models that split the flow are refused with a note.
Guide last updated:
For scripts and AI agents
- JSON API:
POST https://twinworkslinecapacity.usefulweb.app/api/v1/calculate(same calculation as this page) - Interface description: OpenAPI 3.1 · AI guide: llms.txt
- MCP server (Streamable HTTP):
https://twinworkslinecapacity.usefulweb.app/mcp· tool nametwinworks_line_capacity
Example request
curl -s -X POST https://twinworkslinecapacity.usefulweb.app/api/v1/calculate \
-H 'content-type: application/json' \
-d '{"stations":[{"name":"Saw","cycle":40,"availability":95,"performance":95,"quality":99},{"name":"Mill","cycle":96,"parallel":2,"availability":85,"performance":90,"quality":98},{"name":"Deburr","cycle":38,"availability":98,"performance":95,"quality":100}],"availableMinutes":450,"demand":400,"wip":20}'